ADU Strategy in Seattle

An accessory dwelling unit, or ADU, is a smaller second home on the same lot as your main house. In Seattle it can be a converted basement, an attached unit, or a detached backyard cottage, which the City calls a DADU. Deciding whether to add one is as much a real estate decision as a building decision.

Before you talk to a builder, it helps to know how an ADU could change your property's value and your options for renting or housing family. As your agent, Henos Adhana works through those questions with you so the plan matches your budget and timeline.

This page covers buying a home with ADU potential, adding a unit to a home you own, and selling a home that already has one.

What an ADU can do for you

An ADU can bring in rent, house an aging parent or an adult child, or give you a separate home office. An attached unit shares the main structure. A detached cottage has its own walls and entrance, which usually means more privacy, a larger construction job and a space that reads like a small house. The right choice depends on your goals, so we start by naming them: income, family, resale, or a mix.

From there, we look at your lot and neighborhood to see which of those outcomes is realistic and what the trade-offs are. Not every property is a strong candidate, and knowing that early saves money.

Seattle's rules, as of October 2026

Seattle's rules changed twice in the last two years. Since June 30, 2025, a Seattle lot can have two ADUs in addition to the main house, attached or detached, each up to 1,000 square feet. Since mid-January 2026, Neighborhood Residential zones allow up to four homes on a lot, and more on large lots. Within a quarter mile of a major transit stop, or when at least two homes are affordable, the limit is six. ADUs count toward that total, need no parking, and cannot be split off from the main house by subdividing the lot.

State law sets the floor for cities in urban growth areas, which includes the Eastside and north-end cities. Under RCW 36.70A.681, from House Bill 1337 of 2023, a city must allow at least two ADUs per lot and cannot cap ADU size below 1,000 square feet. It cannot require the owner to live on the property, and it cannot require parking within half a mile walking distance of a major transit stop. Each city still writes its own details.

Lots differ. Steep slopes and other critical areas, setbacks and lot coverage can limit what fits. We review your lot together, and Henos connects you with designers and permit professionals for the details. The City also offers pre-approved backyard cottage designs through its ADUniverse site.

Sources: City of Seattle, ADUs and middle housing (rules effective June 30, 2025), City of Seattle OPCD, updated Neighborhood Residential rules (January 2026), RCW 36.70A.681, accessory dwelling units, Seattle SDCI, accessory dwelling units

Buying a home with an ADU in mind

Some buyers want a home that already has a unit, so a family member can live close or the space can be rented. Others want a lot with room to add one later. When we tour, we look past the main house at the yard, alley access, slope, existing structures and where a second unit could sit.

If a home already has a unit, we check whether it was permitted as an ADU before you count on using it as one. ADU potential can be a fair reason to favor one property over another, and we factor it into how we judge homes and write your offer.

Selling a home with an ADU, or deciding whether to build

A permitted ADU appeals to buyers who want rental income, room for family or a separate office. When you sell, we present the unit clearly, with accurate details about its size, permit status and how it has been used, so its value is understood rather than overlooked.

If you own the home and are weighing a build, an ADU is one of several options. We compare it with selling, remodeling or staying put, and look at the likely return against your timeline before you commit. If you plan to rent the unit in Seattle, it generally must be registered with the City's rental registration program, and Washington's landlord rules apply.

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Common questions

What is the difference between an ADU and a DADU?

An ADU is any accessory dwelling unit, including attached units and converted basements. A DADU is a detached unit, such as a standalone backyard cottage.

How many ADUs can I have on a Seattle lot?

As of October 2026, two, in addition to the main house, and each can be up to 1,000 square feet. They count toward the total number of homes the lot allows.

Can I add an ADU to my Seattle home?

Many lots allow one, but it depends on your zone, lot size and site limits such as slopes and setbacks. We can review your property together to see whether it is a realistic candidate.

Do I have to live on the property to rent out an ADU?

No. Washington law bars cities in urban growth areas from requiring the owner to live on the property. In Seattle, a rented unit generally must be registered with the City's rental registration program.

Does an ADU add value to a property?

It can, through added living space and potential rent, though the effect varies by neighborhood and by how the unit is built and used. We look at your situation rather than a general rule.

Should I build an ADU before selling?

Not always. Building is a large investment, so we weigh the likely return against your timeline and goals before recommending it as part of a sale.

How do we get started?

Reach out to Henos Adhana for a conversation about your property and what you want the ADU to do, and we will map out the next steps from there.

ADU Strategy