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Building an ADU in Seattle: a guide to the current rules

Seattle is in the middle of a building reform that’s reshaping what homeowners can do with their land. Since 2023, state law has required cities across Washington to allow accessory dwelling units — detached cottages, garage conversions, and attached in-law units — on most lots that previously allowed only a single house. Seattle has implemented these changes through its Department of Construction and Inspections (SDCI), and a homeowner today has significantly more options than they did even two years ago.

This guide summarises the rules as they stand in mid-2026: what’s allowed, where, at what size, and what the permitting process looks like. It draws directly from the published text of Washington’s ADU legislation and Seattle’s municipal code, with links to the primary sources at the foot of the page.

Read this before anything else. This guide is general information, written to help you get oriented. It is not legal advice, it is not professional advice, and it must not be your basis for any building or financial decision. Seattle’s rules have changed substantially several times in recent years, they will change again, and how they apply to any specific lot turns on details — zoning, lot size, critical areas, easements, existing structures, utility capacity — that no general guide can account for. Treat this page as a rough starting map, nothing more. Before you commit money to an ADU project: verify the current rules with SDCI directly, and put certified, registered professionals between you and every consequential decision — an architect or designer experienced with Seattle’s code, a licensed contractor, and where the situation calls for it, a land-use attorney. If anything on this page and the advice of a qualified professional ever disagree, believe the professional.

What counts as an ADU

An accessory dwelling unit is a self-contained living space — with its own entrance, kitchen, and bathroom — on the same lot as a main house. Seattle recognises two broad kinds.

An attached ADU (AADU) is inside or attached to the main house: a basement apartment, a converted attic, an addition with its own entrance.

A detached ADU (DADU) is a separate structure — what Seattleites usually call a backyard cottage. A converted garage counts. A tiny house counts too, provided it sits on a foundation; a tiny house on wheels does not (the city treats those like camper trailers, and you can’t live in one on a residential lot in Seattle).

Since Seattle’s 2025 code updates, ADUs can also be configured in stacked arrangements — two units in one detached structure, for instance — which gives lots more flexibility than the traditional “one cottage in the backyard” picture.

Understanding the current rules is easier with a little history, because Seattle’s ADU rules are the product of three distinct waves of reform.

The 2019 city reforms. Seattle moved early. In July 2019, the city removed several of the biggest local barriers: it allowed two ADUs per lot instead of one, removed the requirement that the property owner live on-site, and eliminated off-street parking requirements for ADUs. These reforms are why ADU construction in Seattle accelerated years before the rest of the state caught up — by 2023, Seattle was permitting more ADUs than single-family houses.

The 2023 state legislation. In what observers called Washington’s “Year of Housing,” the state legislature passed two companion bills, both effective July 23, 2023. House Bill 1337 deals specifically with ADUs: it requires cities and counties planning under the Growth Management Act to allow at least two ADUs per lot where single-family homes are allowed, forbids them from capping ADU size below 1,000 square feet, prohibits requiring street improvements as a condition of an ADU permit, restricts parking mandates near transit, and requires that ADUs be allowed to be owned as condominiums. It also declared that restrictive covenants created after the law’s effective date — the kind homeowners’ associations use — cannot prohibit ADU construction. House Bill 1110, the “middle housing” bill, is broader: it requires cities to open formerly single-family zones to duplexes, triplexes, townhouses, and similar housing types. It matters for ADU purposes because the two laws interlock — ADUs can count toward the housing density HB 1110 requires cities to allow.

Seattle’s 2025 implementation. Seattle’s compliance legislation — Ordinances 127211 and 127219, implementing HB 1337 and HB 1110 respectively — took effect on June 30, 2025, with further permanent citywide zoning legislation following later that year as part of the One Seattle Plan. The most useful practical change for anyone reading the rules: ADU standards that previously varied confusingly from zone to zone are now consolidated in a single section of the municipal code — SMC 23.42.022 — and apply consistently across all zones that allow residential development. The same wave of legislation reshaped what a residential lot can hold overall: on lots that existed as of June 30, 2025, up to four dwelling units are now permitted — counting the main house and ADUs together — and up to six near major transit stops or where affordable units are included. (Lots created after that date follow different rules — one of many details where your specific situation matters.)

If you remember one citation from this guide, make it SMC 23.42.022. That is where Seattle’s ADU standards now live, and it’s the section any professional you hire will be working from.

What you can build

How many. A lot with a principal dwelling can have up to two ADUs — two attached, two detached, or one of each. Both detached units can even share a single structure. And under the 2025 density rules, most existing Neighborhood Residential lots can now hold up to four dwelling units in total (counting the main house and ADUs together), configured flexibly — which means the two-ADU allowance is now a floor on your options rather than a ceiling. For most homeowners, though, the classic configurations — a backyard cottage, a basement apartment, or both — remain the practical starting point.

How big. The gross floor area of an ADU may not exceed 1,000 square feet under the current code. That number is more generous than it first sounds, because several things don’t count toward it: underground stories (or portions of stories) are excluded entirely, as are up to 250 square feet of an attached garage, storage areas accessed only from outside, and a modest allowance for bicycle parking. There’s also a meaningful exception for conversions: an attached ADU may exceed 1,000 square feet if the part of the building it occupies already existed as of July 23, 2023 — good news if you’re converting a large basement or wing of an older house. One helpful technicality for Neighborhood Residential lots: ADU floor area is exempt from the lot’s overall floor-area-ratio limits in NR1, NR2, and NR3 zones, so building a cottage doesn’t eat into the floor-area budget for the rest of your property. One grey area to flag honestly: council amendments in late 2025 considered allowing larger ADUs in some cases (for example, units with three or more bedrooms), and we have not been able to confirm the final adopted state of those changes — if you’re contemplating anything over 1,000 square feet, confirm the current limit with SDCI or your designer before you plan around it.

How tall. This is one of the bigger recent improvements for backyard cottages. Under the old rules, DADUs were held to substantially lower height limits than houses. Under the consolidated standards, ADUs in Neighborhood Residential zones get the same height limit as a principal dwelling — 32 feet, with a pitched roof ridge allowed to extend up to 5 feet above that. That makes two-storey cottages far more feasible than they used to be. Other zones follow their own limits; your lot’s zoning designation determines the number.

Where on the lot. ADUs follow the same yard and setback provisions as principal dwellings, with a rear-yard allowance: a detached unit can sit in the rear yard provided it stays at least five feet from any lot line that doesn’t abut an alley — and where a lot line does abut an alley, no setback is required at all on that side. A detached unit also needs a minimum separation from the main house (five feet, eave to eave, in Neighborhood Residential zones; three feet elsewhere).

Converting an existing outbuilding. If you have a garage or other accessory structure that existed before July 23, 2023, the code goes out of its way to make conversion feasible: conversions are permitted even where the existing structure doesn’t meet current lot-coverage or setback rules, and SDCI’s Director can grant waivers to smooth the path. The structure still has to be brought up to habitability standards — this is a “the city wants your old garage to become a home” provision, not a free pass on safety.

Parking. None required. Seattle has not required off-street parking for ADUs since 2019, and the current code states it plainly. One caveat: if your lot has existing required parking, you can’t simply demolish it for the ADU unless you replace it somewhere permissible on the property.

Owner-occupancy. Not required. Since 2019, you don’t have to live on the property yourself — you can rent out both the main house and the ADU(s).

Other things worth knowing. ADUs are exempt from street-improvement requirements and from Mandatory Housing Affordability contributions — two costs that apply to many other kinds of development. Under the current code, an ADU can be owned and sold as a condominium unit; the mechanics of condominium creation are genuinely complex and firmly in “talk to a professional” territory. And building an ADU can trigger modest tree-planting requirements if your lot doesn’t already meet the city’s minimum tree standards — a small line item, but one that surprises people.

Where ADUs are allowed

The short answer: nearly everywhere houses are. The consolidated standards permit ADUs in all zones where single-family dwellings are permitted — the Neighborhood Residential zones that cover most of the city’s residential land, plus Residential Small Lot, the multifamily Lowrise/Midrise/Highrise zones, Neighborhood Commercial, and others.

The longer answer involves the exceptions. Lots in the Shoreline District and in historic districts remain subject to their overlay rules. Lots containing environmentally critical areas — steep slopes, wetlands, riparian corridors — face additional restrictions, and the density rules are calculated differently on such lots. Every lot has its own particulars: easements, sewer capacity, tree protection. This is exactly the territory where a general guide runs out and site-specific professional advice takes over — please treat it that way. SDCI offers pre-application guidance, and the city’s ADUniverse tool (next section) includes a feasibility search for your specific address; both are worth using before you spend money on design, and neither replaces a professional’s review of your actual lot.

ADUniverse: Seattle’s pre-approved plans

One of Seattle’s genuinely distinctive offerings. The city’s Office of Planning and Community Development maintains ADUniverse, a program built around a gallery of pre-approved DADU designs created by local architects. Because the city has already reviewed the building plans, permitting a pre-approved design is dramatically faster and more predictable than permitting a custom one — in most cases a permit issues in a matter of weeks rather than months.

The trade-off is flexibility: you’re choosing from a set menu rather than designing from scratch, and the plans still need site-specific review (foundations, utilities, and placement are always particular to your lot). But for a homeowner whose priority is a straightforward backyard cottage without a drawn-out design process, it’s often the fastest path from decision to construction. ADUniverse also includes a step-by-step process guide and the address-level feasibility tool mentioned above.

The permitting process

Every ADU needs a permit — an unpermitted unit is a liability that will surface at sale or refinance, and legalising one after the fact is its own permit process. In outline, the path looks like this:

Feasibility. Confirm what your lot’s zoning allows (ADUniverse’s search tool, SDCI’s resources, or a design professional). This is where lot-specific constraints surface.

Design. Either a custom design (architect/designer) or a pre-approved ADUniverse plan.

Application. For a new DADU or an AADU created within an existing house, you’ll apply for a construction addition/alteration permit through SDCI. Legalising an existing unauthorised unit requires a construction permit to establish use. Electrical work may involve separate service applications through Seattle City Light.

Codes and charges. The unit must meet current Seattle residential, building, mechanical, electrical, and energy code standards. During the application you’ll also complete a King County sewer capacity charge acknowledgment — King County levies a sewage treatment capacity charge on new units, and it’s a real ongoing cost worth pricing in early.

Review, then inspections. SDCI reviews the plans (fast for pre-approved designs, longer for custom), then the build proceeds through the standard sequence of construction inspections to final approval.

Timelines vary with application volume and project complexity; pre-approved plans measured in weeks, custom designs typically measured in months. Pre-application conferences with SDCI are available for complex projects and can surface problems while they’re still cheap to fix.

What it costs

Costs deserve their own guide — see our Seattle ADU cost guide for permit fees, the King County sewer charge, and what typically drives total cost. The honest short version: a new-build detached cottage in Seattle is a significant construction project — typically well into six figures — while conversions of existing space (basements, garages) can come in substantially lower. Two cost categories consistently surprise homeowners: utility connections (bringing water, sewer, and power to a detached structure can be a major line item, occasionally rivalling the structure itself on difficult sites), and the King County sewer capacity charge mentioned above. Get itemised bids from more than one builder, and make sure utilities and fees are inside the number, not beside it.

Recent changes, and what’s still moving

It’s worth being direct about this: Seattle’s ADU rules have changed three times in six years — 2019’s city reforms, the 2023 state legislation, and the 2025 compliance-and-rezoning wave — and the most recent changes are new enough that their practical effects are still settling. Fee schedules adjust annually. Council amendments continue to fine-tune specifics like size allowances, and at least one of those specifics (larger units for three-plus bedrooms) sits in a grey area this guide has flagged rather than guessed at.

That churn is mostly good news for homeowners — each wave has expanded what’s possible — but it means any written guide, this one included, ages. We update this page as the rules move, and each revision is dated. Where this guide and the city’s current published rules disagree, the city is right.

The disclaimer, one more time — because it matters

We opened with this and we’ll close with it, because it is the single most important thing on this page. This guide is a rough starting point, not an authority. It cannot see your lot, your zoning overlay, your sewer line, your slope, your trees, or your budget. It is not legal, architectural, engineering, or financial advice, and no decision — hiring, designing, borrowing, building — should rest on it. Before you act: confirm the current rules directly with SDCI, and engage certified, registered professionals — a designer or architect experienced with Seattle’s code, a licensed contractor, and a land-use attorney where the situation warrants one. The rules summarised here are real, but their application to your property is exactly the kind of question that professionals exist to answer, and the cost of good advice is trivial next to the cost of building the wrong thing.

Sources

Last reviewed: July 2026. This guide is general information only — not legal or professional advice. Rules change; verify anything consequential with SDCI and with certified, registered professionals before acting on it.