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ADUs and additions

ADUs and additions in Skagit, Snohomish, Island and Whatcom Counties

An accessory dwelling unit (ADU) is a second home on the same lot as your house: a basement or garage apartment, a backyard cottage, or a unit built onto the house. An addition gives the home you already have more room. We design and build both.

This page explains what Washington's 2023 ADU law changed, shows what we found for cities in our four counties, and walks through how a project runs with us. It shows no prices. Your city or county decides approvals and schedules, and at the visit we check what applies to your lot.

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Washington's 2023 ADU law, in plain words

Facts checked November 12, 2026. Reviewed by Shawn, who owns Washington Construction.

This is a plain-words summary, not legal advice. The statute and your city's code control.

What it is. In 2023 the Legislature passed House Bill 1337 (Chapter 334, Laws of 2023). It took effect on July 23, 2023 (final bill report). The rules now sit in RCW 36.70A.680 and RCW 36.70A.681. The state Department of Commerce reviews local plans against them (RCW 36.70A.682) and explains them on its ADU page and in its guidance for local governments (September 2023, updated June 2024).

Where it applies. Inside urban growth areas, in cities and counties that plan under the Growth Management Act (RCW 36.70A.680(2)). Our four counties and the cities in them are on that Act's plan-update schedule (RCW 36.70A.130(5)).

What a city or county must allow (RCW 36.70A.681(1)):

  • Two ADUs on a lot, in any zone inside an urban growth area that allows single-family homes: one attached and one detached, two attached, or two detached.
  • Detached ADUs, and an ADU on any lot that meets the minimum lot size for the main home.
  • Conversions of existing buildings, such as a detached garage, even if the building doesn't meet current setback or lot-coverage rules.
  • An ADU size limit of no less than 1,000 square feet, and a roof-height limit of no less than 24 feet unless the main home's own limit is lower.
  • Setbacks, yard coverage, tree retention, entry-door location, appearance and design review that are no stricter than the main home's.
  • A detached ADU at the lot line where the lot line meets a public alley, unless the city or county routinely plows snow on that alley.

What a city or county can't require (same section):

  • That the owner live on the lot. Commerce's page notes one exception, for ADUs used as short-term rentals.
  • Impact fees on an ADU above half of the fees charged for the main home.
  • Public street improvements as a condition of permitting an ADU.

Parking (RCW 36.70A.681(2)). No off-street parking can be required within a half-mile walk of a major transit stop. Elsewhere a city or county can't require more than one space per ADU on a lot under 6,000 square feet, or more than two per ADU on a larger lot. There are narrow exceptions, including near large commercial airports. "Major transit stop" is defined in RCW 36.70A.696.

What the law leaves to cities and counties (RCW 36.70A.680(4) and (5), RCW 36.70A.681(3) and (4)):

  • The ADU rules don't apply to lots with critical areas or their buffers, or to certain drinking-water watersheds.
  • A city or county doesn't have to allow an ADU where other laws limit building because of on-site sewage systems, critical areas or similar site problems.
  • It can restrict short-term rental use of an ADU, apply health, safety, building-code and environmental permitting rules, apply development rules that apply to everyone, and prohibit ADUs on lots that aren't connected to public sewer.
  • It can limit a very small lot (2,000 square feet or less) to two ADUs plus the main home.

Outside urban growth areas. Under a 2026 law (effective June 11, 2026), a county may, but doesn't have to, allow a detached ADU on a rural parcel under added limits: one ADU per parcel, proof that water and septic can handle it, and a size cap of 1,296 square feet at most, among others (RCW 36.70A.685).

When it applies in your city. Each city and county writes these rules into its code at the same time as its next periodic comprehensive plan update (RCW 36.70A.680(1)). The statute sets those updates for December 31, 2024 for Snohomish County and its cities, and December 31, 2025 for Skagit, Island and Whatcom Counties and their cities; some very small towns in the 2025 group can take up to two more years (RCW 36.70A.130(5) and (6)). Where a city or county hasn't updated its code, the state rules override any local rule that conflicts with them (RCW 36.70A.680(1)).

Your city or county sets its own rules. The state law sets minimums and limits. Your city or county writes and applies the rest: utility connections, septic, critical areas, the building code and the permit process. Rules differ from place to place, and they change. The visit covers what applies to your lot. No permit, approval or schedule is promised here; the city or county decides.

How the main cities have adopted it

We read each source below on November 12, 2026, and each entry shows the date of the source. Codes change, so call your planning department for the current text. "We have not confirmed" means we could not find or open a current source, not that the place hasn't adopted the rules. Plan updates were due December 31, 2024 in Snohomish County and December 31, 2025 in Skagit, Island and Whatcom Counties (RCW 36.70A.130(5)).

Skagit County

  • Mount Vernon. Adopted. Code updated to match the state law: up to two ADUs per lot, up to 1,000 sq ft, owner occupancy not required. The city also lists interim ADU rules (Ordinances 3909 and 3918). Source: Skagit Council of Governments packet, May 5, 2026; city code-amendments page, read Nov 12, 2026
  • Anacortes. Adopted. Code updated to match HB 1337; owner occupancy not required; ADUs exempt from impact fees. Source: Skagit Council of Governments packet, May 5, 2026 (link above)
  • Burlington. Partly confirmed. Lot-size, tenure and parking requirements for ADUs were removed; the packet doesn't say how many ADUs a lot may have. The city's Title 17 as adopted May 15, 2023 allowed one ADU per primary dwelling. Source: Skagit Council of Governments packet, May 5, 2026; city Title 17, adopted May 15, 2023
  • Sedro-Woolley. Pending. Two ADUs per lot, up to 1,000 sq ft, is listed as pending; the city is considering pre-approved ADU plans. Source: Skagit Council of Governments packet, May 5, 2026
  • La Conner. Two ADUs allowed per single-household lot. Source: Skagit Council of Governments packet, May 5, 2026
  • Skagit County (unincorporated). Draft. The county's April 2025 notice of its development-code update proposes two ADUs in urban growth areas with sewer. We could not confirm adoption. Source: County notice of availability, April 21, 2025

Snohomish County

  • Everett. Adopted. The City Council approved code changes allowing two ADUs on each qualifying lot. Pre-approved ADU designs were announced June 23, 2026. Source: HeraldNet, July 24, 2023; HeraldNet, June 23, 2026
  • Lake Stevens. Adopted. Code section LSMC 14.44.045 allows two ADUs per lot, attached or detached, with one parking space per ADU. MRSC called the city's new ADU rules a good match for the state law. Source: City ADU submittal checklist, approved 04/2026; MRSC, April 15, 2025
  • Snohomish County, unincorporated. Adopted. The County Council approved an ADU ordinance on March 19, 2025. A county planning briefing said the state deadline for jurisdictions in the county was July 1, 2025. On rural lots the county allows one ADU per lot, with water and septic checks. Source: HeraldNet, March 26, 2025; Snohomish County Tomorrow briefing, July 17, 2025
  • Stanwood. Proposed. A middle-housing ordinance that includes two ADUs per lot went to a Planning Commission hearing on August 11, 2025. We could not confirm adoption. Source: City hearing notice, July 29, 2025
  • City of Snohomish. Draft. A new ADU chapter (14.170 SMC) would allow up to two ADUs per lot, and the Planning Commission discussed it. We could not confirm adoption. Source: City draft chapter; Planning Commission materials, Feb 4, 2026
  • Marysville. Adopted. The city's FAQ says two ADUs are allowed on a lot that meets its zone's minimum lot size, attached or detached in any mix, and that the owner need not live on the lot. Source: City ADU FAQ, read Nov 12, 2026
  • Arlington. Pending. A 2026 ADU update to the zoning code (chapter 20.44 AMC) went to the Planning Commission. We could not confirm adoption. Source: Planning Commission staff report, May 22, 2026

We have not confirmed the current code for Monroe.

Island County

  • Coupeville. Adopted, per the town's project page: a Town Council meeting on April 22, 2025, an "Adopted Resolution + Code" entry, and project completion in June 2025. The project covers middle housing and HB 1337. Source: Town of Coupeville project page, read Nov 12, 2026

We have not confirmed the current code for Oak Harbor, Langley and Island County.

Whatcom County

  • Bellingham. Adopted. The City Council approved the ordinance on December 8, 2025, and it took effect December 23, 2025. Up to two ADUs per lot in any configuration; owner occupancy not required; no parking required (Ordinance 2025-12-35). Source: City ADU ordinance summary, file dated Feb 13, 2026
  • Ferndale. Draft. The city's chapter 18.34 draft provides for two ADUs per lot. We could not confirm adoption. Source: City draft, dated Feb 12, 2024
  • Whatcom County. Adopted, per a news report: on December 9, 2025 the County Council voted 5-2 for zoning amendments that bring county code in line with the state ADU rules. Source: Whatcom Watch, February 2026 issue
  • Lynden. Draft. A 2025 ordinance (25-1708) in a City Council agenda packet would allow two ADUs per lot. The copy is unsigned, and we could not confirm adoption. Source: Council agenda attachment, 2025

We have not confirmed the current code for Blaine.

The routes we build

  • Detached ADU: a backyard cottage or a unit over a garage.
  • Attached ADU: an apartment built onto the house, such as an in-law suite.
  • Conversion: a garage, basement or other existing space turned into a home.
  • Addition: more room for the house you have, such as a bedroom, a bath or a primary suite.

What shapes each job, and none of it shows up as a price here: the lot (slope, access, trees), the utilities (sewer or septic, water and power, how far they are and whether the lines have room), the house it joins (roofline, siding, floor heights and the systems in the walls), the size and the finishes.

How a project runs with us

  1. Visit

    We look at the lot with you: where a unit could go, how water, sewer or septic, and power would reach it, and what is already there to convert. We take photos and measurements.

  2. Your city or county's rules

    We go through the rules that apply to your lot as far as we can see them, and tell you which questions to take to the planning department. You decide whether to go on.

  3. Design

    If you have plans, we work from them. If not, we draw layouts and 3D views in-house, and bring in the designers and engineers the project needs.

  4. Written scope

    A budget broken into line items by phase, in writing.

  5. Permit application

    The drawings and the application go to your city or county. They set the rules and make the decision.

  6. Build

    Trades are chosen from competing written bids, each trade works under a written purchase order, and every change is a signed change order. You get progress updates with photos, and billing follows the schedule in your proposal. If there is a construction loan, we keep the documents your lender asks for at each draw.

  7. Final walk

    We walk it with you and hand over the documents and manuals.

Residential electrical can stay with us under our license WASHIC*743DE, which is residential, from the panel through trim-out. Plumbing is done by certified plumbers.

Pre-approved plans

Some cities offer pre-approved ADU plans. Everett announced three pre-approved ADU designs on June 23, 2026 (HeraldNet), and Sedro-Woolley told the Skagit Council of Governments it is considering them (May 5, 2026). A plan program can have its own rules, so ask your planning department before you choose a design. Tell us if you want to start from one, and we'll say at the visit whether it fits your lot.

What it costs

This page shows no prices. For dated public figures and what moves the cost, see the cost guides. We price your project in writing after we have seen the lot.

Owner-builder and construction loans

Want to act as your own builder? We manage the trades and the paperwork: owner-builder project management. Working with a construction lender? New homes, ADUs and additions explains the documents we keep for each draw.

Questions people ask

Can you build an ADU on my lot?

It depends on your lot and on your city's or county's rules. At the site visit we look at the lot with you and talk through the questions to take to your planning department.

Do I have to live on the property?

State law says a city or county can't require the owner to live on the lot (RCW 36.70A.681(1)). Commerce notes an exception for ADUs used as short-term rentals. Check your city's rules.

Can I rent an ADU for short stays?

A city or county may restrict short-term rental use of an ADU (RCW 36.70A.680(5)). Check your city's rules before you plan on it.

Can I build an ADU if my home is on septic?

Maybe. A city or county may prohibit ADUs on lots that aren't connected to public sewer, and outside urban growth areas a county can require proof that the septic system and water supply can handle another home (RCW 36.70A.680(5), RCW 36.70A.685). We look at this at the visit.

Will the state law let me build whatever I want?

No. It sets minimums and limits for city and county rules. Your city or county sets the rest and decides on your permit. No permit, approval or schedule can be promised ahead of that decision.

Can you work with my lender?

Yes. We keep the documents lenders ask for at each draw: purchase orders, change orders, lien waivers, dated photos and the budget against actual. Your lender's rules govern the draws, and we help you meet them.

How much does an ADU cost?

We don't show prices here. The cost guides show dated public figures and what moves the cost, and we price your project in writing after we have seen the lot.

Start with a visit

Tell us in the project box: your town and county; the rough size of your lot; any garage, basement or other building you'd like to convert; what the ADU is for (family, a rental, an office or something else); and any timing you have in mind. We'll say at the visit what we can do.

Town only. We never ask for a street address here.

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A person reads every request and gets back to you to set up a site visit.

What happens next

  1. A person reads your request.
  2. We contact you by phone or email, whichever you prefer.
  3. We visit and talk through what goes into a written proposal.

Prefer to call?

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Sources and dates

All read November 12, 2026 unless a date says otherwise.