ADU and DADU Rules in Seattle: A Homeowner's FAQ

Seattle went from one of the harder big cities to add a backyard cottage in, to one of the easier ones. If you own a single-family lot here — or you’re buying one partly for its ADU potential — the rules are friendlier than most homeowners assume, and the details matter. Here are the questions we hear most, answered plainly. One caveat up front: code details change, and the Seattle Department of Construction and Inspections (SDCI) is the source of truth for current requirements. Treat this as the map, not the survey.

What’s the difference between an ADU, an AADU, and a DADU?

ADU (accessory dwelling unit) is the umbrella term for a second, self-contained home on a lot — its own kitchen, bath, and entrance.

• An AADU (attached ADU) is inside or attached to the main house — most often a basement apartment or an attic conversion.
• A DADU (detached ADU) is a separate structure — the classic backyard cottage.

How many ADUs can I have on a Seattle lot?

Seattle’s reforms allow up to two ADUs on most single-family (neighborhood residential) lots — for example, a basement AADU plus a backyard DADU, or two attached units. That’s the headline change that made Seattle notable: one house can legally become three homes on a single lot. Lot size, lot coverage, and setback rules still apply, so not every lot can physically fit a DADU. Verify what your specific parcel allows with SDCI before you spend money on design.

Do I have to live on the property?

Seattle removed its owner-occupancy requirement as part of the same reform package. You can own a house with two ADUs and rent all three units without living there — which is why investors now read these rules as closely as homeowners do. (Short-term rental use is regulated separately and more strictly; check the city’s short-term rental rules before planning an Airbnb.)

Do I need to add parking?

Seattle largely eliminated the requirement to add off-street parking for an ADU. Practically, your tenants and your block still have to absorb the cars, so think about it even though the code mostly doesn’t force you to.

What does it cost to build a DADU?

Honest answer: a wide range, and anyone quoting you a single confident number before design is guessing. The big cost drivers are size, site conditions (slope, trees, sewer and power connections), finish level, and whether you use a custom design or a pre-approved plan. Seattle maintains a gallery of pre-approved DADU plans that can cut design time and permitting friction — a genuinely useful program if your lot suits one of the plans. Get multiple builder bids, and budget meaningfully beyond the construction contract for utility connections, permits, and site work.

How long does permitting take?

Faster than it used to be, slower than you want. Pre-approved plans speed the building-permit side; site-specific issues (environmentally critical areas, trees, side sewer capacity) slow everything. Talk to SDCI early — a pre-application conversation is cheap relative to a redesign.

Can I sell a DADU separately from the main house?

Washington’s legislature has been pushing cities toward more ADU flexibility statewide, including pathways toward separate ownership in some circumstances — but whether and how a specific lot can be split or condo-ized is exactly the kind of question that needs a current answer from SDCI and a real estate attorney. Don’t underwrite a purchase on the assumption you can sell the cottage separately until a professional confirms it for that parcel.

What rent can an ADU earn?

It depends on size, finish, light, privacy, and neighborhood — a polished one-bedroom DADU near light rail rents very differently than a low-ceiling basement studio. Rather than anchoring on a number, study actual listings for comparable units in your area for a month. And remember Seattle’s landlord regulations are among the most detailed in the country (registration, just-cause rules, move-in cost limits) — read them before you become a landlord. Our guide to basement apartments and rental income goes deeper on the landlord side.

Does an ADU add resale value?

Generally yes, but less cleanly than the rent math suggests. Appraisers value ADUs unevenly — some give substantial credit, some treat a DADU as a glorified outbuilding — and your buyer pool both expands (house-hackers, multigenerational families) and narrows (buyers who don’t want a tenant relationship). A permitted, legal unit with documented rental history appraises and sells far better than an unpermitted conversion. If you’re buying specifically to offset your mortgage with rent, our piece on house hacking in Seattle compares the ADU route against buying a duplex outright.

I’m buying a house that already has an ADU. What do I check?

• Permits. Pull the permit history. An unpermitted unit isn’t worthless, but it carries legalization cost and risk, and you should pay accordingly.
• Legal unit status. Confirm the unit was established as an ADU, not just finished space with a hotplate.
• Side sewer and utilities. Two or three units on one old side sewer is a load question — a sewer scope is cheap insurance.
• Rental compliance. If it’s tenant-occupied, confirm registration and get the lease terms in writing; you inherit the tenancy.

The bottom line

Seattle’s ADU rules are now genuinely permissive: up to two ADUs on most lots, no owner-occupancy requirement, little to no parking mandate, and a pre-approved plan program to lower the barrier. The constraints that remain are physical (your lot) and financial (construction costs and appraisal uncertainty) more than legal.

If ADU potential is part of why you’re buying — or you’re selling a house whose cottage is half its value story — work with an agent who has actually transacted ADU properties, and know what that agent charges before you sign anything. On Manaky Homes, local agents publish their fees and pricing models openly so you can compare before you commit; the marketplace is in its waitlist phase, and early access is free.