You’ve probably heard of ADUs (accessory dwelling units), or perhaps seen one on a neighborhood walk. They’ve gained popularity across the country in recent years and are now part of the fastest-growing segment of the national housing market, especially in west coast cities where the cost of living is increasing. Here in Seattle, a change to residential building regulations in 2019 has made ADUs more accessible to homeowners than ever. 

In this series, we’re sharing highlights from conversations we’ve had with Katherine Pelz, a Certified ADU Specialist as well as a licensed architect in Seattle.

DADU by Katherine Pelz Architecture

What is an ADU?

The simplest definition of an ADU is a scaled-down secondary living space within a residential home or property—simple because, in most cases, individual cities have detailed definitions for ADUs based on size, building codes, and other specifics. (In this series, we’ll be focusing on the City of Seattle’s ADU codes). 

There are two types of structures within the ADU umbrella: attached ADU (AADU) and detached ADU (DADU). An AADU is a separate living space within a property’s primary structure (like a basement apartment or in-law suite), while a DADU is a secondary structure separate from the main home (a guest house above a detached garage or a backyard cottage, for example). 

Here in Seattle, ADUs can have up to 1,000 square feet of living space, which must include a kitchen, living and sleeping areas, bathroom, and must have a lockable entrance to create privacy from the main home. And, thanks to recent changes to ADU regulations in Seattle, property owners can now have up to two ADUs on a single lot.

Here’s an overview of Seattle’s ADU regulations:

The nitty gritty: Seattle's ADU specifications
  • Must be 1,000 square feet or less
  • Must have living, sleeping, kitchen, and bathroom spaces and a lockable door
  • Cannot house more than eight residents (unless they’re related to the property owners)
  • If you have (2) ADUs on a property, one must meet green building standards or be reserved for income-eligible housing.
  • Parking is not required, but you cannot remove any existing off-street parking to build unless you replace it somewhere else on the property.
  • If the ADU is attached to the main house, sound- and fire-proofing is required between the two units, or you must have an interconnected, hard-wired fire alarm system.
  • Electrical panels and temperature controls must be located within the ADU or in a common area accessible to both units.

Codes and regulations for each type of ADU are specific and not limited to the list above—if you’re interested in learning more, we recommend referencing the City of Seattle’s ADU hub.

AADU by Katherine Pelz Architecture

Are ADUs a good investment? 

ADUs come with many benefits, not only to the homeowner and ADU occupants but also to the neighborhood and city at-large—especially in a city like Seattle. 

For a homeowner, an ADU can provide a source of passive income—which can help offset mortgage payments—or a comfortable and private space to house guests or family members. Much of the non-monetary value lies in the little things—having someone to help you take out the trash or shovel the driveway when it snows. Katherine, who has an ADU attached to her own home, says that it’s brought her a greater sense of social community and comfort knowing she can house a loved one or rent it out as life situations change. 

Beyond the personal benefits ADUs offer, they bring long-term benefits to cities and communities. Denser city neighborhoods mean we can use existing transportation infrastructure (instead of using tax money to build out), create less need for car commutes, and stabilize the city housing market. They can also be a way to provide more housing in neighborhoods without imposing large multi-family housing developments.  

There are important questions to ask yourself if you are considering building an ADU: Do you want to share your space? How much do you want to spend on the improvements? A well-designed space with lots of privacy for each unit can require thoughtful planning. Who do you want to live in your new space? Is the neighborhood going to meet the criteria of who you want to rent to, or have in the space? 

One of the biggest challenges on an ADU project can be navigating codes. Every lot is different, which means there are different unknown factors with every ADU project (working with easements, for example). Depending on your specific priorities for building an ADU, there are a number of ways to go about it, from hiring an architect for a custom design to using one of the City of Seattle’s pre-approved plans and building some of it yourself.

Ultimately, building an ADU is personal, and success depends on creating something that fits your lifestyle and goals. Whether an ADU is a good investment is a question only you can answer, and hopefully, this overview gives you a helpful starting point if you’re considering this type of project. 

I’m interested in building an ADU—where should I start?

Consult an expert. With any project, we always recommend talking to a trusted expert before getting started. ADU Specialists are uniquely positioned to help homeowners and developers understand site eligibility, local regulations, development process and costs, and the return on investment of ADUs.To learn more, visit Katherine Pelz Architecture. 

Read up on city regulations. The City of Seattle has a library of helpful resources for homeowners and contractors. Visit Seattle's ADU information hub to learn about city regulations, pre-approved ADU plans, and more.

Stay tuned. We’ll be posting more ADU tips and information from our conversations with Katherine in the coming weeks.