Can I Build an ADU (Accessory Dwelling Unit) in Washington?

ADU permit requirements. State law, internal vs. detached, basement apartments, and impact fee waivers.

Often yes — many states now require cities to allow at least internal ADUs.

In Washington

WASHINGTON HAS ONE STATE BUILDING CODE IN FORCE EVERYWHERE, ADOPTED BY REFERENCE: "Except as otherwise provided in this chapter, there shall be in effect in all counties and cities the state building code which shall consist of the following model codes which are hereby adopted by reference:" IN ALL COUNTIES AND CITIES - the model codes named in that section are the code, without any act by your city. AND YOUR LOCAL GOVERNMENT MAY CHANGE IT, BUT ONLY IN ONE DIRECTION: "The governing bodies of counties and cities may amend the codes enumerated in RCW 19.27.031 as amended and adopted by the state building code council as they apply within their respective jurisdictions, but the amendments shall not result in a code that is less than the minimum performance standards and objectives contained in the state building code except as provided in subsection (2) of this section." NOT LESS THAN THE MINIMUM. So a local amendment may add to the state code and may not fall below it. That is the opposite arrangement from states that cap local rules at the model code, and it means a stricter local requirement in Washington is normal rather than suspect. FOR A HOUSE THERE IS A SECOND GATE, AND IT IS THE PART MOST READERS WOULD NEVER GUESS: "Except as provided in subsection (2) of this section, no amendment to a code enumerated in RCW 19.27.031 as amended and adopted by the state building code council that affects single-family or multifamily residential buildings shall be effective unless the amendment is approved by the building code council under RCW 19.27.074 (1)(b)." SHALL BE EFFECTIVE ONLY IF APPROVED BY THE BUILDING CODE COUNCIL. A local amendment touching single-family or multifamily residential buildings is not merely constrained - it does not take effect at all until a state council approves it. If you are told a local residential rule differs from the state code, that approval is the thing to ask about. THE STATUTE NAMES ONE EXCEPTION TO THE FLOOR, AND ONLY ONE - a county or city "may adopt amendments that eliminate any minimum gross floor area requirement for single-family detached dwellings or that provide a minimum gross floor area requirement below the minimum performance standards and objectives contained in the state building code." MINIMUM HOUSE SIZE IS THE ONE PLACE A LOCAL GOVERNMENT MAY GO BELOW THE STATE CODE. It may eliminate a minimum gross floor area for single-family detached dwellings altogether. Quoting the floor above without this exception would state the opposite of the statute in the one case the legislature carved out. BUT THE STATE CODE DOES NOT FIX THE NUMBERS YOU ACTUALLY DESIGN TO. The climatic and geographic design criteria - ground snow load, wind speed, seismic design category, frost line depth - are set by each city in its own code, and neighbouring Washington cities do not agree. STANWOOD AND AIRWAY HEIGHTS DISAGREE ON EVERY ONE OF THEM. Stanwood amends the International Residential Code table to a "Ground Snow Load 25 (roof snow load must also be 25psf unless proven otherwise by the structural engineer of record)", a "Wind Speed 110 mph", "Seismic Design Category D2" and a "Frost line depth 12"". Airway Heights publishes a "Ground Snow Load*: 39 lbs/ft2", a "Wind Speed (Gust): 85 mph", "Seismic Design Category: C" and a "Frost Line Depth: 24"" - with its own footnote that the "Minimum roof snow load to be 30 lbs/ft2 in the City of Airway Heights." READ THE LABELS, NOT ONLY THE NUMBERS. Airway Heights states its wind figure as a GUST; Stanwood's is unqualified. So the two cities do not even record the same measure, which is a further reason a figure cannot simply be carried across - you would be comparing one city's gust to another city's unlabelled speed. SO A FIGURE CARRIED FROM ONE WASHINGTON CITY TO THE NEXT WILL BE WRONG, and it can be wrong in either direction: Stanwood's ground snow load is LOWER than Airway Heights' while its stated wind speed is HIGHER. A plan drawn to one city's criteria is not a plan that passes in the other. AND ONE OF THOSE FIGURES IS NOT A FIXED NUMBER AT ALL. Stanwood's roof snow load is a default an engineer may displace - 25 psf "unless proven otherwise by the structural engineer of record". The way out is evidence, not discretion, and the code says so. ASK YOUR CITY FOR ITS DESIGN CRITERIA BEFORE YOU DRAW ANYTHING. They are published in the municipal code, not in the state code, and this guide cannot give them to you for your address.

State Law May Require It

A growing number of states have passed laws requiring cities to allow at least internal ADUs (within the existing footprint of the home — basement apartments, above-garage units, etc.). Many cities also allow detached ADUs, though rules on size and placement vary. Check whether your state has an ADU-enabling law before assuming your city can simply say no.

Types of ADUs

Internal ADUs include basement apartments, converted attic space, and attached additions. Detached ADUs include backyard cottages, converted garages, and purpose-built accessory structures. Each type has different permit requirements and setback rules.

Key Requirements

Most ADUs need: a separate entrance from the primary dwelling, adequate parking (usually one additional off-street space), fire separation (1-hour rating for internal ADUs), and compliance with all building codes for habitable space. Basement ADUs require egress windows in every bedroom.

Reduced Impact Fees

Some states limit impact fees for ADUs, making them cheaper to permit than a full second dwelling. Many cities waive or significantly reduce impact fees for ADUs. Ask your city about current fee schedules — this can save thousands of dollars.

In many states you can build an ADU thanks to state law requiring cities to allow it. Internal ADUs are the easiest path. Start by checking your state's ADU law and your city's specific ADU ordinance.

Permit fees by city in Washington

Verified from each city’s published fee schedule. Where a city has no separate fee listed for this project, open the city page for the current requirements and amount.

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Permit fees by city in Washington
	Permit fee	Source
Bonney Lake	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
Camas	See city page for the current Accessory Dwelling Unit (ADU) fee	Accessory Dwelling Unit (ADU) →
Des Moines	Set by the City Manager or designeeVerifiedvia Residential building permit	Residential building permit →
Edmonds	Set by council resolution; not in codeVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Issaquah	$33.26 minimum, tiered by valuationVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Kenmore	Set by adopted fee scheduleVerifiedvia Residential building permit	Residential building permit →
Lake Stevens	Set by city council resolutionVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Longview	Set in the master fee scheduleVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Lynnwood	Set by city council fee ordinanceVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Maple Valley	Set by Development Fee ScheduleVerifiedvia Residential building permit	Residential building permit →
Mercer Island	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
Moses Lake	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
Mountlake Terrace	Set by annually adopted fee scheduleVerifiedvia Residential building permit	Residential building permit →
Pullman	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
Puyallup	Set by city manager executive orderVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Sammamish	See city page for the current Residential Building Permit fee	Residential Building Permit →
SeaTac	Set by City of SeaTac Fee ScheduleVerifiedvia Residential building permit	Residential building permit →
Tumwater	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
University Place	Set by city council fee resolutionVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Walla Walla	From $30.00, tiered by valuationVerifiedvia Residential building permit	Residential building permit →
Wenatchee	Not published in code; ask the cityVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →

See the national overview for this permit →