Guide to Developing New Lots in King County

The Challenges of Developing Land in King County

“What can go wrong?”

(my former boss)

When I moved from the city to the country - unincorporated King County - designing a home for a client became a far more complicated problem. Undeveloped lots face a number of challenges just to become feasible to build. If you are buying new property in King County, or if you already have, this guide will help you sort through the madness.

Before you buy a new lot, ask for up to 60 days to perform a feasibility analysis. Hire an architect to walk the property and lend their professional expertise to a number of issues, from providing water and sewer, to working around environmental areas, to evaluating the zoning code of the local jurisdiction. You want to know the dark secrets before you buy.

Here is an outline of the chief issues with new lots:

#1 Septic

When a connection to a sewer system is not available, a rural lot cannot support a new home unless there is a feasible septic system. In King County, there must be enough area for both primary drain field and an equally sized reserve area. This often means you must have about 3,000 sq. ft. available. The first thing I do when evaluating a new lot is to hire a septic designer to perform feasibility.

Green Tip: Although composting toilets may be approved (the State of Washington keeps a list of approved systems) they do not lessen the septic area requirement, because the assumption is that a future owner may not want the hassle of a composting system.

The septic designer will need to do some soil test pits to evaluate the soil. They may also need to dig some test holes and allow rainwater to naturally fill them during the rainy months to prove to the King County Department of Health officials that the soil is infiltrating as needed.

The drainage field must obey setbacks from environmental zones like streams and wetlands, and cannot be placed on steep slopes (of 40% or more, a drop in 4 feet for every 10 feet). However drainage fields may often be placed within unbuildable areas adjacent to steep slopes. You can get a preliminary idea for the slopes by looking up your lot on the King County IMap.

#2 Water

If you have a rural lot, it is possible that you may have a connection to district water. On islands in which there is a single-source aquifer, water may be scarce. Even if there is a connection to district water, is important to know if you have a water share, and if your share has already been paid for (usually there is a substantial fee at the time of connection). Even if you have the right to purchase a water share, you may be waitlisted in perpetuity.

If you don’t have district water, you will need a well. Look at the size of your lot; if it is over 5 acres, you are golden, but if not, you will need to know when your lot was created. If it was created prior to 1972, you are still allowed to drill a new well. If not, you have a problem.

Some lots come with an easement (referenced on your deed) that allows you to connect to a Class B well that serves multiple lots. These wells are substantially more expensive to create and maintain because they must provide water that meets a minimum level of quality, a max threshold on contaminants like arsenic. For single family wells, your septic designer can submit the paperwork. But for a shared well, you will need to hire a well engineer.

New wells also require a 100 foot setbacks from property lines (unless you obtain an easement) and all man-made structures (except the pumphouse). Driveways are OK. The total area of a 100 foot circle is 31,450 sq ft (3/4 acre). This eats up a large, contiguous area of a rural lot. The well head must be accessible to a drill rig, so you may need to build a temporary driveway with quarry spalls (large crushed stones).

Green Tip: A friend tells me that it is now feasible in King County to approve a rainwater catchment system as a primary water source in lieu of a new well. I have to see it to believe it.

The well and on-site septic applications must be submitted to King County Public Health but need not be approved yet to submit your building permit application. The permit cannot issue until a verified water source is in place.

#3 Environmental Hazards

When you acquire a new lot in King County, look for areas on or adjacent to the site mapped as “critical areas” such as slopes, rivers & streams, wetlands, landslide hazard areas, and so forth. Even if nothing is mapped, you will need to hire a geotechnical engineer and/or an environmental consultant to walk the site and provide a report to the county saying so.

Sometimes, the consultant will find something, and it is important for them to take special care in delineating exactly what is going on, and how big it is, as their interpretation is often accepted by King County reviewers. At which point the county issues a “Critical Area Designation” (CAD) that spells out the requirements for a project.

Note: I have seen CADs issued by King County that reference “further investigations” before making a commitment to a setback. Take this as a warning. You do not want to start a project without a fully unambiguous setback.

King County has an obligation to allow private land owners to utilize their land. When the setbacks from environmental features interfere with buildable areas on the lot, there is a process for obtaining reductions in setbacks in exchange for mitigation measures. I have found this process, called: “Critical Area Alteration Exemption” to be cumbersome, very slow to navigate, and expensive.

Note that drainage fields are governed by King County Public Health, and have a separate set of requirements to those governed by King County zoning for buildings. They also have a different way of classifying wetlands and streams.

Here are some environmental hazards:

Wetlands

Wetlands are sneaky. A field that is bone dry in July may have standing water in the winter. The resulting soggy soil may support wetland plants and be a protected zone with buffers up to 100 feet or more. I can often spot a wetland but it is not always obvious.

Rivers & Streams

When I was 10 years old, I fell in love with Frank Lloyd Wright’s Fallingwater House, perched over the Bear Run River. Long gone are the days when we could build over streams, or even near streams. We have discovered that this is bad. Protected corridors around rivers and streams protect the water quality that allows life to thrive. Even seasonal streams may have buffers of 50-100 feet or more.

Slopes

Slopes require setbacks. If you are adjacent to a slope, you will need to hire a geotechnical reviewer to test the soil, sometimes with a drilling rig. Often, the most beautifully scenic bluffs are those most worrisome. Some may require setbacks of up to 65 feet. Note that a survey is very useful as you can easily delinate on the contour map where steep slopes exist.

Landslide Hazard

Similar, but not identical to slope hazards, are landslide hazard areas. Geotechnical engineers will look for long-term shifting of the soil and offer recommendations. You want these recommendations to be in your favor, and a good geotech will both be frank about the concerns as well as offer you the best recommendation possible given the nature of the site.

seismic hazard

Another soil issue, seismic hazards are often more common in urban areas in which previous grading activity has moved earth around. This “fill dirt,” if it is not properly compacted, will liquify during earthquakes. That is not good.

You may require piles or pin piles as part of your foundation design. These are metal columns driven into the earth that support your building directly on bedrock. Additional concrete may also be needed to create grade beams spanning between piles. All of which increases the cost of the foundation.

#4 Restrictive Easements

Before you buy property, read your deed. If you don’t understand it, ask someone to help you interpret it. If you don’t read your deed, you may be surprised that someone has loaded your property with restrictive easements that serve other lots. I have seen a lot loaded with easements to provide space accommodating wells and drainage fields for three adjacent parcels. The owners were clueless.

How could this happen? The real estate agent was clueless, and there is no professional standard requiring them to be anything else. This is why it is so important to have your architect perform a feasibility study on a new lot.

#5 Soil Infiltration

All the water that hits a man-made surface, such as a paved road, or a roof, must be dealt with according to the rules laid down in the King County Stormwater Manual (KCSM). The cheapest way of treating this water is to disperse it. However, dispersion may require a “flow-path” of 100 feet of gentle slope that is designed to remain naturally vegetated, and these areas must have setbacks from drainage fields.

Other kinds of systems will directly infiltrate stormwater into the ground. The ability to do so depends on the type of soil. You may need to have the a geotech perform a “pit” infiltration test to determine the infiltration rate. For sites that poorly infiltrate, you may be faced with costly requirements for underground water detention.

Green Tip: Green roofs naturally slow down the rate at which water flows off the roof, and they can be utilized by the KCSM for avoiding costly underground detention.

For sites that are highly constrained by environmental hazards, early coordination with a civil engineer is important to understand feasibility.

#6 Fire Sprinklers

Sprinkler systems are often required in rural projects. If your driveway is too long, to narrow, or if it doesn’t have a good turn-around for the fire truck; or the distance from the parking area to all sides of all structures is more than 150 feet, then you will likely need sprinklers. Further, the drag line from the nearest hydrant cannot be too long. Sprinklers will likely add about $25,000 to a project, and you may need to incorporate storage and pumps into your well pumphouse, or provide for tank storage if the flow rate from your water service is not sufficient to meet “fire flow.”

In Conclusion

Before purchasing an undeveloped lot in King County, have a professional perform a feasibility analysis that addresses all of these issues. Most sellers will recognize the need for a period of 30 to 60 days for this process.

In order to help get a sense for the up-front “soft” costs of developing land, let’s imagine that you recently bought a 5-acre parcel with some wet areas and some slopes. You wanted to button up all the unknowns prior to designing a new home. The land cost $200,000 for a scenic, forested parcel.

$5,000 Architect Feasibility Study & Site Planning
$5,000 Survey
$5,000 Wetland Delineation
$2,000 Critical Area Designation Fees
$5,000 Infiltration Testing
$5,000 Septic Design Fees and KCPH Submittal
$5,000 Geotech report

$32,000 Subtotal

At this point, you will still need to design a home for your lot. For a $1M custom home, fees for these services might constitute:

$50,000 Architecture
$10,000 Interior Design
$7,500 Structural Engineering
$7,500 Civil Engineering

$75,000 Subtotal

To relate these to the value of your $1M construction project, you are spending an additional 20% on the land, 3.2% on feasibility, and 7.5% on design and engineering.

At AirMod Architecture, we don’t love developing land, but we do love helping our clients realize their dreams. As a result, we take the land development process very seriously. Our feasibility report is our warranty that your project can be built.

Sincerely,

Brett Holverstott

Guide to Planning a Backyard Cottage

Is a backyard cottage right for you?

A backyard cottage is an opportunity for homeowners to leverage the development potential of their lots, while helping to ease our region’s housing crisis. A cottage is a great opportunity to house a family member or generate some rental income. The downside is the high cost for all construction projects in our region, the tight constraints on space, and the complex zoning requirements.

This guide is an introduction to the complex feasibility issues of designing a backyard cottage. Although zoning varies based on your jurisdiction, we focus here on Seattle and propose a simple grading system that will allow you to begin the conversation with your architect.

In Seattle's housing crisis, can backyard cottages help families reconnect? (Read Article)

Recent zoning changes make it easier to build these dwellings. They could mix up the modern middle-class American life.

introduction

An accessory dwelling unit (ADU), by code, contains everything you need to live. Whether an unit attached or unattached to the main house, an ADU is triggered by a planned cooking area - specifically an outlet that is capable of supporting an electric range.

Attached units in new homes are required to have a separate lockable entry, separate circuit breakers and temperature controls, and be separated from the main residence by 1-hour fire rated construction. You do not need to meet these requirements for remodels such as basement conversions, but they must still meet some basic building code items like ceiling height minimums.

Detached accessory dwelling units (DADU’s), or backyard cottages, are required to be separated from the main residence by at least 5 feet of air space, preferably 10 feet to avoid fire-ratings on the exterior walls. Cottages have special allowances: they are allowed to intrude within the rear yard zoning setback. But when they do, they are subject to more strict height limitations. We will walk through these.

I have designed or permitted five cottages in Seattle (see the Sunset Hill Cottage and the Beacon Hill Cottage), and clients love them. But a word of warning - the zoning requirements are very complex, and can trip up even established architects who do not take the feasibility seriously.

the rules

Seattle’s rules for single family lots allow attached or detached ADUs up to 1,000 square feet. You can even do both, if you go green, or provide affordable housing units. Attached garages do not count toward this total, and parking spaces are not required for the units.

lot coverage & lot width

To determine whether your lot is feasible, you will first make some calculations relying on your lot coverage and lot width.

First you will want to get access to your lot information. I suggest locating your property on the Seattle GIS Map. If you click on your property it will link through to the Seattle Parcel Data, and the GIS also has a convenient measure tool for obtaining your lot dimensions. Alternatively you could walk outside with your tape measure.

Step 1: Check minimum requirements.

A backyard cottage in a single family lot may be feasible if it meets the minimum requirements, including a lot size of at least 3,200 SF, a lot width of at least 25 feet, and a lot depth of at least 70 feet. There is an exception to the lot depth requirement if your lot is over 5,000 sf. If you do not meet these requirements, your lot receives an “F” grade.

Step 2: Know how your lot width affects building height.

The width of your lot directly effects the height of your allowed backyard cottage. Two terms to know:

Base height: the height of the structure at the exterior face of the wall.

Ridge height: the height of the highest point on the roof. This is an extra allowance, over the base height, and determined by the shape of your roof.

Lots are broken into the following categories:

Lots less than 30’ wide. The base height is only 14’, which makes a one-story unit more practical. The ridge may extend 3’ higher. Add a green roof or green building standard for another 2’ of height. (Lots less than 30 feet wide will probably prove infeasible for other reasons such as lot coverage, which we will get to next.)

Lots 30’-40’ wide. The base height is 16’, which is the minimum feasible for a two-story unit. The ridge may extend 7’ for a pitched roof, or 4’ for a shed roof. Plus the 2’ green bonus.

Lots 40’ -50’ wide. The base height is 18 feet, which is a comfortable 2-story structure. The ridge may extend 5’ for a pitched roof or 4’ for a shed roof. Plus the 2’ green bonus.

Lots more than 50’ wide. The base height is 18 feet, which is a comfortable 2-story structure. The ridge may extend 7’ (the only change) for a pitched roof or 4’ for a shed roof. Plus the 2’ green bonus.

Step 3: Calculate your available lot coverage.

In single family zones, you are typically allowed a lot coverage of 35%. This means you can cover 35% of your lot with structures. If you have a small lot with a large house, you may be very close to your maximum lot coverage, although if your lot is under 5,000 sf you are allowed 1,000 sf + 15%. Please note that we are not talking about building areas on upper stories, just the footprint of the structures on the ground. This includes occupied structures, garages, sheds, and so forth.

To calculate lot coverage, multiply your lot size by 0.35 and subtract all the existing coverage. Ignore any coverage you plan to demolish, like that old shed that has lost the battle with the blackberry bushes.

(Lot Size * 0.35) - (House Coverage) - (Garage Coverage) - (Shed Coverage) … = Remaining Coverage.

You might find the first floor area of your house online, but you can also run a tape around your house and take dimensions, measuring from the exterior face of your foundation wall, and calculating the area. Whatever remains is your available lot coverage, which has a big impact on your score. If you have zero coverage remaining, your lot receives an “F” grade.

Step 4: What’s my preliminary grade?

In general, the wider your lot the better, and the more available lot coverage the better. I have developed a grading system based on what kinds of solutions you might find if you are trying to maximize the development potential of the lot. Let’s start from worse case to best case.

D Grade lots

Single story cottages with an optional sleeping loft.

There are not very many lots less than 30 feet wide that have available lot coverage. However, if you have a few hundred feet available, the 14’ base height with only a few feet for the ridge will most likely constrain your project to a one story structure. However, you could think about a sleeping loft, and if it is not more than 200 sf, it will not require a code-compliant stairway or ceiling height. Substitute a ships ladder to save space.

C grade lots

2 story cottages, or carriage houses (over a garage), with a footprint of about 400 sf.

For lots greater than 30 feet wide, if you have around 400 sf of available lot coverage, you can build a 2-story DADU of around 800 sf, with a 16’ base height. This is the minimum size that I would recommend for a 3-bedroom unit. In this kind of unit, there is a shared kitchen and dining area, and one of the bedrooms can be used as a den. These units are also the some of the most desperately needed affordable housing in Seattle for families with children.

If your lot is more than 40 feet wide, you get a C+ for the height bonus to 18 feet. If you have less than 400 sf of available coverage you may still be able to salvage a two-story structure with up to two bedrooms.

When a garage is desirable, lots with about 400 sf of available lot coverage can also accommodate a “carriage House,” in which a unit is built over a detached garage. To keep the garage ceiling low, consider an out-swing garage door instead of an overhead door.

B Grade lots

2 story cottages that maximize the 1,000 sf allowed area, with an optional attached garage. Consider a rooftop deck for added privacy.

If you have at least 500 sf of available lot coverage, you can build 2-story backyard cottage that maximizes the allowed 1,000 sf development potential. With another 250 sf, you can attach a garage, depending on driveway access.

To offer the occupant more private space, consider adding a rooftop deck to the garage. This is feasible if the area of the deck is outside any of the side and rear setbacks.

If your lot is more than 40 feet wide, you get the height bonus to 18 feet, so upgrade to B+.

A grade lots

Flexibility to build 1,000 sf on the ground floor plus an attached garage, but enough height to comfortably fit a 2-story structure.

If you have 1,000 sf of available lot coverage, you can build a backyard cottage entirely on the ground story, unless you prefer a 2-story structure. With an additional 250 sf, you can attach a garage. If your lot is more than 40 feet wide, you get an A+.

This is the most flexible position for a property owner but you often see more compact cottages in order to keep the backyard available for the main residence. With an A grade, you will have a lot of design flexibility.

setbacks and rear yard area

Score high on the scorecard? Great, but it is only a starting point. Now let’s see if you have space to build.

Step 5: Do you have room enough to build outside setbacks?

View fullsize

Things are tight in a backyard! As you draw your available building site, know the following:

A cottage will need to be a minimum of 5’-0” feet away from the main residence, but preferably 10’-0” or else you will need to respect building code restrictions for fire protection.

A cottage will need to be a minimum of 5’-0” from side lot lines, unless you secure an easement from your neighbor. However, being so close to the lot line may still require temporary shoring during excavation, unless you have a shallow foundation such as a slab on grade. Give yourself as much space as possible.

A cottage will need to be a minimum of 5’-0” from the rear lot line, unless you are on an alley. If you are on an alley, the setback is 0’-0”, but in reality 1’-6” gives you the minimum needed to get foundations and drainage in.

The entrance to the unit will need to be at least 10’-0” from the lot line, if it is facing a side lot line.

Step 6: Do you have enough room to build in your rear yard?

In addition to the non-yard area, a cottage is allowed to occupy up to 60% of the rear yard area, but only if you aren’t removing a significant or 2’-0” diameter trees.

The rear yard area is the area between your rear lot line, and your rear setback line. The rear setback line is measured from the rear lot line and is the lesser of 25’-0”, or 20% of your lot depth, with a minimum of 10’-0” for very small lots. But here’s the rub - if you already have structures in your rear yard that you intend to keep, they take away from what you may use for the cottage.

To add complexity to this, if you are on an alley, you calculate your rear yard area as the area between the centerline of the alley, and your rear setback line. So it might give you a bonus of around 150 sf in your rear yard.

If you have defined a building plot outside of these setbacks that allows you to use most (or all) of your available lot coverage, you are in business. That is, so long as you also don’t have any issues with the following…

other site considerations

Some of the items on the laundry list that need to be considered include the following:

Step 7: Do you have any critical areas?

If you use the Seattle GIS Map, locate your parcel, and click through to the Seattle Parcel Data you should be able to view any critical areas mapped on the property, such as wetlands, riparian areas, steep slopes, landslide hazards, and “seismic hazards” (code for unstable fill dirt.) You can even view these in the GIS map if you turn on the right layers. Any of these can throw a project into jeopardy, with buffers and setbacks that can be considerable.

Step 8: Do you have any significant trees?

Seattle wants to preserve its urban forest and will make you respect buffers to large, healthy trees that are “significant.” You are only allowed to impact a portion of the area within the drip line. If they are unhealthy and dangerous, you may obtain permission to remove them with an arborist report. Seattle will only give you a rear yard area bonus of 20% if you are working around significant trees. (Also note, that cottages are now subject to new tree planting/preservation requirements.)

Step 9: Does your property have a slope? How much?

You think your site is flat? It is almost never, ever, flat. And Seattle will not take your word for it, especially if your project is knocking on the door of the height limits, which is why a topographical survey is almost always required with a permit submittal. Even slight slopes will govern the height of your foundation walls, excavation, the complexity of the concrete work, shoring issues, and drainage issues. Flat sites, on the other hand, may have drainage issues that will require the services of a civil engineer.

On lots with significant slopes, you may have other options available to you such as terraced garages. And a portion of underground stories does not count against the floor area limits.

Step 10: For new garages, is it feasible to create a new driveway & curb cut?

If you are on an alley, accessing a garage is easy. If you already have driveway access to the street, great. But if you need to create a new driveway from the street (usually on corner or flag lots), you must consult the code. If you need a second curb cut, it will need to be allowed based on your street frontage, and be clear of obstacles such as light poles, power poles, and street trees.

utilities

You might imagine that since there is already a house on your lot, utilities shouldn’t be a problem! But since there is already a house on your lot, there is guaranteed to be a house in the way of construction activity and utility connections. Here is a introduction to some of the potential issues.

Step 11: Is your sewer in good shape?

Seattle must allow you to connect the sewer from a backyard cottage to the nearest branch. This can either tie directly into the waste line in the main residence, or it can tie in to the waste line downstream of the main residence. Best practice is to find a good connection point that has a 6” pipe, but this is not a requirement. Be sure to have your sewer located, scoped, and lined if needed. Know the elevation of the sewer to ensure that you will have positive drainage from the new cottage. If not, you will need a pump. Also note: King County requires a capacity charge for new sewer connections.

Step 12: Do you have any hidden underground utilities?

It is vital to obtain a survey that shows all underground utilities. Have the surveyor call in a utility locate prior to the survey work, so that these will be shown on the survey. Then, before you break ground, you will need to mark them again. Utilities in the street should be shown with their elevations noted. If you discover (worse case scenario) a concrete stormwater culvert running diagonally through your back yard, this may make the entire project infeasible. King County requires setbacks from this kind of infrastructure. (Special note: sometimes a lot will be for sale with serious issues like this - be sure you seek professional guidance before jumping on the last undeveloped lot in Seattle!)

Step 13: Calculate your total new & replaced impervious surface.

City-wide practices for managing stormwater have gotten better over the years, so expect your backyard cottage to be much more of a hassle than the main residence was, twenty years ago. The total area of new impervious surface (roof, new outdoor patio, replaced driveway area, etc) will govern how you are required to manage stormwater.

When your architect submits a preliminary site plan, Seattle will visit your lot and issue a Preliminary Application Report (PAR). This will provide a lot of useful information, especially with regard to stormwater.

Less than 750 sf. You won’t be required to do anything fancy. Splash blocks will allow stormwater to run into the lawn.

750 sf -1,500 sf. You will be required to connect to an approved off-site discharge, such as a culvert or street gutter.

More than 1,500 sf. You will be required to provide on-site stormwater management (OSM) which may include retention or infiltration. This is fancy stuff. To know the preferred strategy you may need an infiltration study and a civil engineer.

Less than 1,500 sf, but no off-site discharge? If there is no approved or accessible way to discharge stormwater, you will be required to provide a bio-retention planter that overflows into a drywell. Neither of these items count against your lot coverage, but planning for them is another thing.

Step 14: Will you require sprinklers?

If you are at the end of a long driveway that is inaccessible to a fire truck, you may be required to provide a sprinkler system, even if the main residence does not currently have one.

prefab feasibility

If there is a good chance you can build on your lot, you may be wondering how you should go about it. I have designed stick-framed cottages, modular cottages, and cottages built from prefabricated panels. Here is an introduction to some of the issues to consider.

What are the options?

Aside from stick framing a house in a traditional way, you can order prefabricated structural insulation panels. SIP’s panels typically come with layers of sheathing bonded to an insulated core. These panels may be small enough for two men to carry, but most often it is valuable to set the panels with a boom truck or crane, rapidly over a day or two.

Alternatively, you can build a custom modular home, in which nearly all work, including interior and exterior finishes, is done at the factory. Site excavation and foundation work must be done before the modules arrive, and the mods are set in one day with a crane. Afterward there is some amount of work that must be done on site, from structural holddowns to finish patching.

The process for building an ADU

The information in this article is intended as an introduction; it is extremely important to have a feasibility analysis performed by a reputable design professional, preferably a licensed architect. Why is this important? If anything is missed early in the feasibility analysis, it becomes the responsibility of the architect to correct the mistake.

During feasibility, you will begin to line up consultants who will be needed for the job, and obtain any reports necessary for the project. Note that although the project may be small, the prep work is usually the same size as that for a larger structure. These are likely to include:

• A survey that includes topography, underground utilities located and mapped, existing structures, easements, and so forth.

• A soils report by a geotechnical engineer. Although is the most loathed expenditure, it is often critical for permit if there are any unusual soil conditions or slopes.

• An infiltration report, also able to be conducted by the geotech, if the impervious areas are likely to exceed 1,500 sf or if there is no off-site discharge.

• An arborist report, if there are significant trees. Other environmental consultants as required.

With enough of this material in hand, design can begin.

Early in the process, the architect should submit a preliminary site plan to the city to begin the process of obtaining a Preliminary Application Report (PAR). In my experience, these are taking two months to turn around, so start them early.

As soon as the idea starts to take shape for the project, the architect can help you line up other consultants that may include:

• A structural engineer.

• A civil engineer. Since Seattle provides a convenient stormwater calculator and standard details, it is much easier to do stormwater design in Seattle without a licensed civil engineer. But there are always subtleties that will require discussions with the side sewer desk in the Seattle DCI.

• A sprinkler designer. Sprinklers may be required if you are at the end of a long private road that makes it difficult for a fire truck to access the house.

• A design-build mechanical engineer.

• For modular: an MEP design consultant(s).

Once construction documents are prepared, they may be submitted for permit.

the next step

I hope you enjoyed this guide. If you have comments or questions, feel free to reach out. If you are intending to move forward with your own backyard cottage project, we would be happy to help.

Introduction to Passive House Construction

What is Passive House?

Passive house construction is a practical building standard for minimizing the energy use of a building. It was developed by asking the question: “how efficient can we get?”

By orienting a building to the south with proper overhangs, by super-insulating your walls, floor, and roof, by air sealing up the building and controlling the ventilation air, and by using efficient systems, you can reduce the energy use of a building substantially (sometimes 80% over code), so that you are only heating or cooling it a few months out of the year in the Pacific Northwest.

The interior of the house will almost always be comfortable - not just the air temperature, but the temperature of all the surfaces with which your body comes into contact. You don’t feel drafts next to windows, even in winter. Walk around naked without getting a chill. (I mean, that’s the dream, right?)

I first discovered passive house construction in graduate school, and I never wanted to build any other way. My first independent design (shown above) was a passive house for friends overlooking Lake Chelan. While designing it, I took the certification training and became an expert, and taught a building crew who had been building houses for 20 years how to do it.

In the decade since, most architects have jumped on the band wagon, and the energy code in Washington State now requires you to choose between various credits that only a few years ago were considered specialized knowledge by Passive house designers and builders. Passive house is no longer weird - it is best building practice.

This guide is a brief overview of what is necessary to make a successful Passive House

Solar Orientation

In the Northern hemisphere, winter sun comes from the south. This makes it very important to point at least half of your total glass area to the south, because during the winter your home needs to soak up that winter sun (whenever it is available).

South-facing sunlight is also easy to control; during the summer, the sun is much higher in the sky, which allows overhangs of a few feet to completely block out summer sun.

Sunlight coming from the East and West is more difficult to control. Light from the North is much more diffuse, making it an excellent choice for art galleries, but doesn’t give you “free heat.”

The biggest challenge with southern-orientation is that you may want views in a different cardinal direction. In this case, you will need to balance south-facing glass with view-facing glass. Overall, you may be surprised with how little glass is required for a successful Passive House. Almost every house has more glass than it needs. (This little design has a lot of glass.)

Heating

In general, heating and cooling systems should be heat pump based. A heat pump is pure electric, but many times more efficient than electric resistance baseboard heaters. This is possible because it moves heat around, rather than generating the heat from electrons. A heat pump can dump cold air outside even when it is very cold out; and it can dump hot air outside even when it is warm.

Heat pumps can be connected to air distribution systems, or individual fan coils (with no ductwork) mounted as floor units, wall units, or ceiling cassettes. Expect the air distribution system to run continuously at much lower air flow than a furnace.

If a concrete slab is a good fit for your project, consider radiant floor heat. Most people choose radiant heat because they like the idea of warm floors. But if you are living in a Passive House, you may not even be aware that the floor heat is on, because the house needs so little heating to remain at a comfortable temperature!

If you want, concentrate your floor coils in the kitchen. Or, provide a south-east facing window in front of an area of the slab you want that warm-floor feel in the morning.

If you do chose slab heating, consider combining it with hot water system (such as a Sanden unit) and keep the radiant floor heat to a simple single-zone design.

Ventilation

You should expect to have a continuous (24 hour) ventilation system in your home to provide clean, fresh air. Even when the air quality outside is poor, it is typically about 10 times worse indoors. Continuous fresh air can also protect you from the off-gassing from materials or indoor pollutants due to cooking. Usually these systems run at very low cfm, so you may not even notice it is running.

In a Passive House it is important to use a heat recovery ventilator (HRV). This transfers heat (or cool) from the outgoing air stream to precondition the incoming air stream, before any additional heating or cooling is applied to that air. If you are using an air distribution system, connect the HRV to air handler. This often reduces by half the amount of energy you spend heating fresh air.

In the Pacific Northwest, we are more often seeing terrible air quality due to summer fires. Consider placing a HEPA filter on the incoming air stream, in an accessible location.

In some of my projects we have used ductless, decentralized HRV units that are inserted into walls throughout the house. These “Lunos” units have the advantage of no ducts, but they do not have good filters for smoke.

Air Sealing

We no longer accept “leaky” homes. There are several reasons for this.

First: air infiltration through the building can carry huge amounts of moisture into wall, floor, and ceiling cavities, and this can degrade the health and longevity of the home. Second: air infiltration causes drafts in windy conditions that make the house uncomfortable.

This is what a blower-door test looks like.

Third: air infiltration costs energy, because all fresh air needs to be heated (or cooled). We want fresh air, but we also want it to be controlled and passed through the HRV.

Air sealing a home often requires the builder to use a wide (2-1/2”) tape to seal up all the joints at the structural sheathing, around the entire exterior of the home. Alternatively, you can use a liquid flashing product to do this (which is a better solution).

After the house has been framed, sheathed, and windows and doors installed, it is always good to perform a “blower door” test to verify the air sealing performance of the home. Best to do this before insulating and drywall, so that you can see where the air is infiltrating and seal it up. You should try to hit an air changes per hour (ACH) of 1 - 1.5 measured at a pressure of 50 Pascals.

You will need to do this again after construction is nearly finished; the energy code may require it if you take this option as an energy credit. There is even a product that will spray into the air during a blower door test to seal up cracks in the drywall, if you need a little extra sealing at the finish line.

Insulation

In a Passive House, more insulation is better. My favorite wall assembly is a 11-1/4” cavity formed by two layers of studs (the “double wall”) and packed full of cellulose insulation. The wall ends up being about a foot thick. This wall works in the Pacific Northwest but not all climates.

To avoid an extra layer of framing, you can build a traditional 2x6 structural wall and then wrap the building with insulation on the outside - be it rigid foam, mineral wool, fiber board, or even cork. Or you can construct your home from foam sandwich panels (SIPs).

A good target for most homes in this climate is R40 walls and floors and R60 roofs. 11-7/8” TJI’s (wood I joists) in the floors get you to about R-40. 14” TJI’s get you to about R-50. You can add a couple inches of rigid insulation on top to get to R-60.

One important part of designing the building envelope (wall, floor, roof) is to understand vapor diffusion. Water should not be allowed through the primary water barrier on the building exterior, but vapor can often transport through the building assemblies. It is often a good idea to use cellulose and fiberboard in combination to allow vapor to permeate through the entire wall. If you use foam, just be sure never to trap moisture in a wall with vapor impermeable materials on both sides.

To Slab, or Not to Slab?

Concrete is an energy intensive building material and a significant contribution to CO2 emissions. But, it has the advantage of durability. If your design is a good fit for a heated concrete slab, plan to insulate it fully with 4-5” of insulation to reach R-30 and provide an under-slab rated vapor barrier.

Crawl spaces are man’s gift to spiders. They work pretty well in the Pacific Northwest, but not most other climates. It may be a less expensive option to provide a crawl than an insulated slab. Be sure to provide a vapor barrier on the floor of the crawl or on the underside of the floor.

Windows and Doors

Getting high-performance windows and doors in the United States is difficult - and by “difficult” I mean expensive. Most domestic manufacturers do not make them, or do not try to sell them to you.

Meanwhile, they are standard in Europe. (Many high-end European manufacturers source them from the Czech Republic, and for many years I have sourced products directly from suppliers there, which comes with its own logistical complications.)

Weco windows - very awesome European products in which the handle for operable units is mounted directly on the glass. Still looking to get them into a project.

Providing high performance windows and doors is the easiest single line-item to chop from the budget, but don’t be tempted. Most of the energy lost from a home is through windows. The better your glass, the more of it you can have, and the more of it you can have facing directions other than South. Further, if the inside surface of the glass falls 7 degrees below the ambient temperature in the home, drafts will form.

I recommend triple pane windows with a U-value of 0.18 and thermally insulated frames. For budget projects this means plastic (PVC). I often provide my clients with an option for a more aesthetically pleasing double-paned wood window with a U-value in the very low 20’s. If you are building a million dollar house, you can prioritize both aesthetics and performance.

I work very closely with my window manufacturers to create a package in which we look for where to save money in the specification of unit sizes. We often go through 2-3 iterations of design on the windows. I rarely bid out packages to multiple suppliers. Find someone you like working with and hone in on the best value for your project.

The Prefabricated House