With today’s affordable housing crisis, it feels more prudent than ever to look to ADUs as one possible solution, especially in Seattle, WA.

According to a case study by Redfin, the median price for a single-family home in Seattle was around $1.1M in March of 2022, compared to roughly $825k four years earlier. That’s an increase of almost 33%! This reality is hardly optimistic, especially for young adults buying for the first time.

On a more positive note, ADUs are quickly gaining in popularity for this very reason. And with new regulations in place from recent years, your options are about to get even better. We’ve seen some truly spectacular ADUs in our time and can’t wait to see what Seattle has to offer!

So, ready to learn more? Let’s dive in!

Are accessory dwelling units legal in Seattle?


In fact, new ADU legislation took effect in August 2019 to address the city’s housing crisis. These reforms removed regulatory barriers, making it easier than ever for Seattle homeowners to add an ADU to their property!

How much does it cost to build an ADU in Seattle, WA?

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The price range to build an ADU in Seattle is generally between $125,000 – $350,000. While $125k is on the low end of the spectrum, it’s likely that your ADU will cost more.

The price of your ADU depends on:

  • Type of construction (above-garage ADUs are typically the most expensive to build, while garage conversions are the cheapest)
  • Size
  • Number of bedrooms and bathrooms
  • Quality of finishes
  • Lot characteristics
  • Availability of material and labor

Homeowners are often shocked by the starting costs for an ADU. But it’s important to understand that building an ADU is not all that different from building a traditional, 2,000-square-foot house. Both require the same elements; ADUs simply have a smaller footprint.

Many people try to use the cost-per-square-foot method to arrive at an estimate for their ADU. But this method is notoriously deceiving, and here’s why.

Any home comes with fixed costs, regardless of size. For example, your design fees will not vary dramatically depending on the size of the ADU. Similarly, all ADUs are required to have a kitchen and bathroom, and that is expensive square footage no matter what. At the end of the day, building an ADU that’s 400 square feet versus 1,200 square feet will not really alter these fixed costs.

With all this in mind, here’s a word to the wise: Be extremely wary of anyone who claims they can build your ADU for under $100k. We monitored hundreds of projects last year and saw the real numbers come through. While there are cost-saving tips to take, it’s unlikely that your project will come in under six digits.

But don’t be discouraged! One great way to keep costs down is by limiting the number of bells and whistles you add. The fancier you get with your design choices, such as vaulted ceilings or top-end appliances, the larger the price tag. It’s worth thinking about your must-haves versus other areas where you could be more flexible.

What kind of ADU can I build in Seattle?

Fortunately, there are so many options at your disposal! The trick is figuring out which one makes sense for you. Each type of ADU has different factors to consider before you make a final decision.

Here’s a quick rundown of your choices.

Detached ADU

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A detached ADU, otherwise known as a DADU or backyard cottage in Seattle, is a stand-alone build that is separate from the primary residence but on the same property, usually situated in the backyard.

From renting out the ADU to housing adult family members or caregivers, there are so many ways to maximize a detached ADU! This option provides a sense of privacy and autonomy while also remaining close to the main house.

Attached ADU

An attached ADU, also known as an AADU or in-law suite in Seattle, differs from a detached ADU because it is connected to the primary residence. They are sometimes wholly or partly created from an existing space, like an attached garage, attic, master suite, or rec room.

Garage Conversion ADU

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This ADU is the most popular option and offers loads of advantages! For homeowners looking to upgrade their dingy garage into usable space, this is the way to go. It’s also financially advantageous given that it utilizes an already existing space. You can either choose to keep your ADU within the footprint of the original garage or expand beyond it.

Over the years, we’ve seen homeowners truly redefine innovation with their creative designs, like these 37 garage conversion ideas you’ll drool over. Everything from adding a deck or sunroom to using the garage door to provide additional natural light are all possibilities in store!

Above-Garage ADU

adu seattle

This is a desirable option for homeowners who don’t want to lose their garage given the valuable storage and parking space it provides. But it’s important to note that most garages aren’t built to withstand the weight of an upstairs unit. So, it’ll need to be structurally enforced, making this option the most expensive type of ADU to build.

Nonetheless, these types of ADUs offer great privacy and amazing views. If you’re willing to shell out the extra cost, above-garage ADUs are excellent additions to any property. Learn if an above-garage ADU is right for you.

Basement ADU

Much like garages, basements have great bones and are more financially feasible options than other, more expensive types of ADUs. This is a fantastic choice for homeowners looking to transform their basements from gloomy to glamorous. Discover how to convert a basement into a living space, and take a tour of this 420 square foot basement conversion!

Where can I build an ADU in Seattle?

State regulations allow an ADU to be built in:

  • Neighborhood Residential (NR) zones, formerly called single-family zones
  • Multifamily zones (e.g., townhouse or row house)
  • Commercial zones (no detached ADUs allowed)
  • Lowrise zones
  • Residential Small Lot (RSL) zones
  • Upland lot within the Urban Residential (UR) shoreline environment (no detached ADUs allowed)

In plain terms? Most Seattle property owners have the opportunity to add an ADU! After all, Seattle’s policy goals from their 2019 legislation changes were to increase both the number and variety of housing choices, with ADUs being one such strategy.

There are a couple of caveats to note:

  • Detached ADUs must be built on a minimum lot size of 3,200 square feet.
  • ADUs are not permitted in apartments in lowrise zones (but they are permitted in apartments in RSL zones).
  • ADUs are not allowed on waterfront lots.

For more information, refer to the Seattle Municipal code.

How many ADUs can I build in Seattle, WA?

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In neighborhood residential zones with a proposed or already existing single-family home, for example, you can build up to two ADUs. You can choose to have:

  • Two attached ADUs
  • One attached ADU and one detached ADU

Keep in mind that a second ADU must meet the green building standard or be a rental unit that is affordable to and reserved for “income-eligible households.”

What is the green building standard? Essentially, it means that your ADU implements certain building strategies and practices that save resources and promote clean, renewable energy. For more detailed additional information on the green building standard, explore this handout provided by the Seattle Department of Construction and Inspections.

On RSL zones and multifamily lowrise zones, however, only one ADU (attached or detached) is allowed for each single-family home, row house, or townhouse unit.

How big can my ADU be in Seattle?

This depends on where you build your ADU.

In neighborhood residential zones, both an attached and detached ADU must be 1,000 square feet or smaller. Fortunately, this excludes spaces like garages, porches, and covered decks less than 25 square feet, and any storage areas that can be accessed from the exterior.

On the other hand, if you plan on building an ADU in a lowrise zone, an attached and detached ADU cannot exceed 650 square feet. Likewise, attached ADUs cannot exceed 650 square feet in a townhouse or row house.

For specific development standards, such as yard and setback requirements, refer to the Seattle Municipal code.

Do I need a permit for an ADU?

adu seattle

Yes. You’ll need to get your building permit approved by the city planning department before you jump into construction.

The time frame for getting your permit approved can be a bit…uncertain. Most ADU projects in Seattle take somewhere between four and six months to get permits but wait times can vary depending on the jurisdiction.

There are also some outlier permits to be aware of. For example, you will need a separate electric permit for any electrical work that needs to be done unless the property owner plans to handle it themself. Oftentimes, electrical circuits need to be altered when establishing an attached ADU.

Similarly, any plumbing work done requires a plumbing permit. And if you plan to connect to an existing side sewer, you may be required to get a side sewer permit, as well.

Even though this process can be time-consuming, never skip it. Not only could you face a lawsuit if a tenant got injured inside the unpermitted ADU, but the city could force you to pay hefty fines if they find out. And the worst-case scenario? You have to tear down your ADU entirely.

Long story short, it’s much better to be patient!

Will my ADU need a parking spot in Seattle?

Good news! You are no longer required to provide a parking space for your ADU.

However, you cannot remove any existing, required off-street parking spaces unless it is replaced elsewhere on the property.

How long will it take to design and build my ADU in Seattle?

adu seattle

On average, ADUs take between 12 – 18 months from design to construction completion.

Surprised? There are a lot of factors that go into making an ADU happen.

There are four main phases: financing, design, permitting, and construction. Each phase has its own timeline, which can sometimes be impacted by factors entirely outside of your control.

Take the supply chain, for example. During the pandemic, there was a huge surge in home renovation projects. This made getting your hands on certain supplies like lumber rather challenging. You might even remember these difficulties firsthand!

Unique lot characteristics can also throw a wrench into things, as well as the backlog of work at your city’s planning department or simply bad weather conditions.

If you’re someone who hates having idle hands, no problem. There are always plenty of things that you can do to help your project keep moving forward. Equip yourself with the necessary ADU knowledge with our E-course, The ADU Blueprint. This comprehensive course will help you build a realistic vision for your project and educate you on navigating complex problems often encountered by homeowners.

Get a free preview of our E-course today!

Can I rent out my ADU in Seattle?

You can most definitely rent your ADU in Seattle! ADUs make great spaces for short-term and long-term tenants in Seattle.

But it’s worth noting that short-term rentals are a bit trickier. If you plan to rent your ADU in the short term on sites like Airbnb, you’ll need a license and to follow the city’s rental regulations. Read more about what short-term rental entails and about how to get started.

Either way, you’ll need to register the ADU under the city’s Rental Registration and Inspection Ordinance (RRIO), which establishes minimum standards to ensure basic maintenance requirements and safe housing in Seattle. Plan on an inspector paying a visit at least once every 5-10 years to verify that your ADU is meeting these standards.

Regardless, opting to rent out your ADU is a great way to earn substantial passive income. So, how much are we talking about?

To give you a better idea, according to rentcafe.com, the average rent for a 691-square-foot apartment in Seattle costs around $2,324 per month. ADUs usually yield higher rents thanks to their privacy and outdoor space, so we’d estimate you could rent your ADU for about $2,600. Not too shabby!

Consider Kylee and Steven’s success story, for instance. In 2017, they found the perfect property to buy. The only problem was its $1.2M price tag, which was especially daunting for a young couple. But they saw light at the end of the tunnel the moment they considered the 400 square-foot ADU above the garage.

The ADU was in dire need of a makeover, and that’s just what Kylee and Steven got to work doing. It wasn’t long before they began renting the new and improved property and generating substantial passive income, allowing them to use their ADU to pay off their mortgage!

Check out the full video tour of their ADU here.

Plan, hire, and manage your ADU project with Maxable!

While times change, the need for housing does not. After consulting with thousands of homeowners and helping hundreds complete ADUs, we’re more passionate than ever about accessory dwelling units!

Building an ADU is a big but rewarding commitment. At Maxable, we’re here to walk you through every step of the way. We’re eager to share our knowledge and connect you to designers and general contractors with proven experience in Seattle. That way, you’ll have the proper support from beginning to end.

Ready to get started? Tell us about your project here to get personalized introductions to ADU professionals in your area.