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The ABCs of Seattle ADUs

Like many cities in the US, Seattle has a need for both more housing and more affordable housing. In the near future, you can expect to see an large uptick in the addition of accessory dwelling units (ADUs) within the city of Seattle as well as other urban growth areas (UGAs) throughout Washington State. Starting in 2025, new rules and relaxed legislation relating to the building of accessory dwelling units and detached accessory dwelling units (DADUs) will take effect. Seattle neighborhoods, and especially their backyards, are probably going to see a lot of changes. 

A guide to Seattle ADUs

So for better or worse, and whether you are in favor of them or not, this article will give you a good overview of Seattle ADUs and the changes that you can expect to see over the coming years.

What exactly is an ADU? 

An accessory dwelling unit (ADU) is a self-contained residential unit that is located on the same lot but usually smaller than the existing single-family home. They allow for independent living within the same property and have their own kitchen, bathroom, living and sleeping spaces. The added living space can either be attached to the existing home in which case it is called an accessory dwelling unit (ADU) or is separate from it in which case it is referred to a detached accessory dwelling unit (DADU). 

What are the different types of seattle accessory dwelling units?

There a few different types of ADUs, primarily based on where they are located on a property as shown in the graphic below courtesy of the Master Builders Association of King and Snohomish Counties:

The different types of Seattle ADUs

ATTACHED accessory dwelling units (AADU) can be either directly attached to the an exterior wall of the existing home like an addition or it can be located internally by converting a basement or an attic space. 

A DETACHED accessory dwelling unit (DADU) is not physically attached to the existing single family home and can either be a free standing unit that, for example, could be added to the end of the backyard or, in some cases, could be added above a garage. DADUs are sometimes referred as a backyard cottage or a carriage house.

The number of ADUs being built is rapidly increasing.

Walking around a Seattle neighborhood like Ballard, you will probably notice ADUs popping up, as if appearing overnight. Well, maybe not overnight but you are not mistaken. ADUs are on the rise (pun intended), and according to the Seattle Times, there are now more accessory dwelling units being built than single family homes. 

Also, there has been an exponential growth in the number of permit applications for ADUs in Seattle as seen in the graph below courtesy of the Seattle Times:

Increase in permit applications for ADUs in Seattle

What are the pros and cons of Accessory dwelling units.

Some of these will depend on which side of the proverbial real estate fence you stand.

The advantages of ADUs and DADUs:

  • Increase in housing supply.
  • Generally are more affordable compared to typical single family homes like houses and townhomes.
  • Allows home owners to age in place and not have to move. For example, owners  can downsize in place by adding an ADU to their property, move into that ADU and then either rent out the house or sell it add to their retirement savings.
  • ADUs are cheaper to build than a single family home because you don't have to buy any land and for AADUs, they can usually tap into the existing plumbing, electrical and sewer systems of the existing home.
  • Generate income by renting out the ADU.
  • Allows for multi-generational living where, for example, the grandparents could live in the accessory unit and their siblings and grandkids live in the main home.

Some potential disadvantages of ADUs.

  • Some of your neighbors are probably not going too be happy that you are adding an ADU especially your immediate neighbors who might be concerned about loosing some of their privacy, natural light and having more people living next door.
  • Space limitations since they tend to be significantly smaller than a regular house so it might take some adjustment for some people.
  • Less privacy since you will be living in close proximity to your landlord or your extended family.  You can't pretend that you're not at home anymore when someone knocks on your ADU front door!
  • As to whether accessory dwelling units actually increase affordability is debatable. Home prices in Puget Sound cities like Seattle, Kirkland and Bellevue are toward the higher end of the spectrum compared to most of the US and even though ADUs are less expensive, can they still be considered "affordable" or are real estate developers just making a quick killing?

10 upcoming changes to seattle and Washington state adu requirements.

Here are some of the main changes to the the building of accessory dwelling units in Seattle and other WA urban growth areas beginning within 6 months of their implementation in January 2025. For some Washington State cities like Seattle, some of these new rules have been in place for a while. As with all changes, some people are going to openly embrace them with both arms and some people probably won't be too pleased.

ADUs will be allowed on all lots zoned single family within cities and within urban growth management zones.

  1. Allows for 2 ADUs per lot, subject to minimum lost sizes and other factors. The added units can be any one of the following combinations: two ADUs, two DADUS or one ADU + one DADU.
  2. There will no longer be a requirement for the owner to live in either the primary residence or one of the ADUs .
  3. Allow ADUs of at least 1,000sf.
  4. A more streamlined permitting process including pre-approved DADU designs.
  5. Duplexes, triples and multi-family structures can add an ADU.
  6. For setbacks from lot boundaries, cities may not impose stricter standards than those for the primary residence. This includes no setback needed for DADUs on lots with alleys.
  7. Must allow addition of ADU conversions of existing structures including basements, attics and above garages, even if those existing structures violate current setback requirements.
  8. The ADUs cannot be used as short term rentals like Airbnb's. The minimum rental period is 30 days. One aim of these new building rules changes is to increase housing supply, not even more short term homes rentals.
  9. Allows for separate sales of individual accessory dwelling units (see requirements below).
  10. Relaxed parking requirements including. Cities may NOT: require any off-street parking for ADUs within half a mile walking distance of a major transit hub, require more than one off-street parking space per unit on lots smaller the 6,000sf and require more than 2 off-street parking spaces per unit on lots greater than 6,000sf.

can I add an adu on my property?

The best answer is consult with a company or architect that specializes in the addition of ADUs to Seattle area homes.

Here are some things to take into consideration that can impact how many ADUs and what size ADUs you can add to  your property.

  • The size of the lot.
  • The zoning.
  • Review the title for the property to evaluate any restrictions including home owner association CC&Rs.
  • The topography of the lot.
  • The type of soil.
  • Does the property have sensitive areas like streams, is in a flood zone or is located in a landslide risk area.
  • How easy will it be to connect the ADU to utilities and sewer? Or a septic system?

Here's a handy site where you can enter your home's address and assess the feasibility of adding an ADU to your property. 

Owners should also take into consideration what impact the addition of an ADU will have on the market value of the existing home since it will use up some of the backyard and reduce privacy. The ADU will definitely add to the overall value of the property but some of that will likely reduce by the value of the existing home. Consult with a Realtor or an appraiser for their opinions on this. Of course, this is probably only relevant if you wanted to sell the primary home separately from the ADU.

How much does an ADU cost to build?

For the Seattle area, on average, an ADU will cost between $300,000 and $400,000 depending on whether it's an attached or detached unit and whether the unit can tap into the utilities of the existing home. However, the price tag can go up for larger ADUs and those with higher end finishes and customizations (closer to around $600,000). That pricing will include connecting to utility services.

Built on-site ADUs versus factory-build ADUs.

You have a couple of options as to how you can add one to a property but the overall cost is about the same. 

Built on-site: here the unit is built from scratch just like a regular home. First you add the foundation, and then the framing and so on.

Factory built: this only applies to DADUs. Here you would choose a design from a selection of options or you can have  a customized home designed and built for you particular needs. The ADU would then be loaded onto a truck (in one piece), and delivered to your property followed by a crane lifting the home onto the newly laid foundations followed by connection to utilities. A crane can deliver the ADU over the top of an existing house and plant it into a hard to access area in your backyard. 

If you order a pre-built ADU, make sure it manufactured to stand up the Pacific Northwest's infamous weather. In other words, don't order one from southern California.

Make sure to get permits when adding an ADU!

If you decide to add an ADU make sure to do it all above board because if you don’t you won’t be able to sell it as ADU. You’ll have to sell it as “bonus space” or “a detached home office”, but not an ADU and you will probably loose some of your investment. Don’t be penny wise and pound foolish and get those permits. Plus buyers might be less inclined to buy the property if the ADU was not permitted.

Can you add an ADU over a garage?

In theory yes, but the majority of garages in the Seattle area do not have a sufficiently sturdy foundation to take the weight of adding living space above it. Plus the slab concrete floor will be a lot thinner compared to that in a house. From an investment perspective, you might be better off just bulldozing the garage and adding a  DADU. Most Seattle garages are full of Costco overspill, two lawnmowers and stuff we never use...but rarely actual cars!

Note that if your garage is sitting right on the property boundary with your neighbor, while it should legally have a feet feet of set back, you will be able add an ADU right plonk on top of that same garage foot print, even if there is no setback from the property line.

Helpful ADU resources:

Washington State Depart of Commerce guidance on ADUs.

City of Seattle construction and permitting.

AAPR information on ADU living.

Conor MacEvilly logo

This article was written by Seattle and Eastside Realtor Conor MacEvilly who has been in the business since 2008. I hope you enjoyed the post and thanks for visiting my website. If you have any questions about Puget Sounds area residential real estate feel free to contact me. I'm happy to help. My direct line (cell) is 206-349-8477.

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