“Washington ADU Laws 2026: Complete Guide for Seattle Eastside Homeowners

*By David Meade, AIA, NCARB | Piper Cole Architects*

> TL;DR: House Bill 1337, signed May 8, 2023 and fully effective July 23, 2023, is the most significant ADU reform in Washington State history. It preempts local restrictions that were previously the primary obstacle to ADU construction on the Eastside. This post explains what the law now requires across Kirkland, Bellevue, Redmond, Sammamish, and Issaquah — and what’s still different city by city. For homeowners who’ve been curious about an ADU, the regulatory barriers are lower than they’ve ever been.

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What HB 1337 Changed — The Complete Picture

Washington’s HB 1337 didn’t just modify ADU rules. It preempted them — meaning local cities that previously banned or severely restricted ADUs are now legally required to permit them under the state floor. Here is what the law mandates for all covered jurisdictions (cities of 25,000+ in King, Snohomish, Pierce, and Clark counties, and cities of 10,000+ in counties above 100,000 population):

Unit allowances:

• At least one attached ADU and one detached ADU (DADU) permitted per single-family lot, regardless of prior local rules
• Both may be rented simultaneously
• Permits for conforming ADUs must be approved or denied within 60 days of complete application

Size and design:

• Minimum allowed ADU size: 1,000 sq ft of floor area (cities cannot set a lower maximum)
• Maximum height for DADUs: 24 ft with sloped roof, 18 ft with flat roof
• Side and rear setbacks: minimum 4 ft in most configurations (cities may be more permissive but not more restrictive)

Process and fees:

• Owner occupancy: cities may not require it as a condition of ADU construction or rental
• Impact fees: ADUs may be charged no more than 50% of the impact fee for a primary unit of comparable size (RCW 36.70A.696)
• Parking: no additional parking required for ADUs within ½ mile of major transit stops

Condominiumization:

• ADUs may be sold as separate condominium units — independent of the primary residence — effective under the 2023 reforms

What HB 1337 does not preempt:

• Critical areas ordinances (wetlands, steep slopes, streams) — these remain fully enforceable
• Design review requirements where they existed for primary residences
• Fire sprinkler requirements (though these are now restricted — see below)
• Tree protection and arborist requirements
• Utility connection requirements and fees

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Fire Sprinkler Reality Check

Fire sprinklers were one of the most commonly cited obstacles to Eastside ADU construction before 2023. Cities routinely required residential fire sprinklers in new detached ADUs, adding $10,000–$25,000 to project cost.

HB 1337 restricts this. Washington cities may not require fire sprinklers in an ADU solely because it is an ADU, unless fire sprinklers are required in the primary dwelling on the same lot.

Most Eastside single-family homes were built before fire sprinkler requirements were common. If your existing home does not have fire sprinklers, your new ADU cannot be required to have them under the 2023 state law.

However: some Eastside cities have argued that certain ADU configurations trigger their fire sprinkler requirements through independent pathways — particularly attached ADUs that share walls with the primary residence. Confirm the sprinkler requirement with your city’s Building Department before finalizing budget. We recommend this as a standard item in pre-application meetings.

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City-by-City Breakdown: What Each Eastside City Now Allows

The following represents current ADU rules as of July 2026. Local ordinances continue to be updated in response to HB 1337 — confirm current standards with each city.

Kirkland

Parameter	Kirkland 2026
DADU (detached ADU) permitted?	Yes
AADUs (attached ADUs) per lot	Up to 1
DADUs per lot	Up to 1
Max DADU floor area	1,000 sq ft
Max DADU height	24 ft (sloped) / 18 ft (flat)
Owner occupancy required?	No
Permit portal	MyBuildingPermit.com
Typical review timeline	4–12 weeks
Fire sprinkler required for DADU?	No (if primary house has none)

Kirkland adopted code amendments in response to HB 1337 in late 2023 and updated further in 2024. The city processes ADU applications through MyBuildingPermit.com along with other residential permits.

Bellevue

Parameter	Bellevue 2026
DADU permitted?	Yes — newly allowed as of July 1, 2025
ADUs per lot	Up to 2 (one attached, one detached, or permitted combinations)
Max DADU floor area	1,000 sq ft minimum allowed (state floor)
Max DADU height	24 ft (sloped) / 18 ft (flat)
Owner occupancy required?	No
ADU condominiumization allowed?	Yes
Permit portal	MyBuildingPermit.com
Typical review timeline	4–8 weeks

Bellevue’s story is the biggest change in the region. Detached ADUs were prohibited in Bellevue until July 1, 2025, when Ordinances 6851 and 6852 took effect in compliance with HB 1337. For the complete Bellevue ADU analysis, see ADU Architect in Bellevue WA.

Redmond

Parameter	Redmond 2026
DADU permitted?	Yes
ADUs per lot	Up to 2
Max DADU height	24 ft (sloped)
Owner occupancy required?	No
Permit portal	MyBuildingPermit.com
Typical review timeline	5–28 business days
Expedited review available?	Yes

Redmond’s ADU code was updated under Ordinance 3186 (January 2025) and Ordinance 3220 (June 2025). Contact permits@redmond.gov or 425-556-2473 for pre-application consultation.

Sammamish

Parameter	Sammamish 2026
DADU permitted?	Yes
ADUs per lot	Up to 2
Max DADU floor area	1,000 sq ft (state minimum applies)
Owner occupancy required?	Not required
Permit portal	Sammamish city portal
Typical review timeline	Varies — 4–10 weeks typical

For ADU-specific guidance in Sammamish, see our ADU Architect Sammamish WA post.

Issaquah

Issaquah adopted ADU code amendments in 2023–2024 in compliance with HB 1337. Permit review timelines run 4–8 weeks for typical ADU applications. Critical areas constraints are significant in parts of Issaquah — the Issaquah Alps, creek corridors, and steep slopes affect substantial portions of the city’s residential land.

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HOA Restrictions: What Can and Cannot Be Enforced

This is the question I get most often from Eastside homeowners: “My HOA says I can’t build an ADU — is that still true?”

RCW 36.70A.696(4), added as part of the HB 1337 package, is clear: homeowners associations may not prohibit the construction or rental of ADUs on lots that would otherwise be permitted by the city. HOA provisions attempting to ban ADUs are unenforceable under Washington state law.

What HOAs can still regulate:

• Design aesthetics that are within a reasonable design review standard (HOAs cannot prohibit, but may require visual compatibility)
• Short-term rental restrictions (separate from ADU construction — HOAs may still restrict nightly rentals on ADU units depending on their governing documents)

If your HOA is citing ADU restrictions as a reason not to build, I recommend consulting a Washington real estate attorney alongside your architect. The HOA may be operating on outdated governing documents that have not been updated to reflect current law.

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ADU Cost Ranges for the Eastside in 2026

Generic ADU cost guides underestimate Eastside construction costs by 20%–40%. Here are realistic ranges for 2026:

ADU Type	All-In Construction Cost
Garage conversion to ADU	$80,000–$250,000
Basement/lower-level ADU	$100,000–$350,000
Attached addition (new construction)	$150,000–$400,000
Detached DADU (600–800 sq ft)	$280,000–$450,000
Detached DADU (800–1,000 sq ft, premium)	$350,000–$650,000

Soft costs — architecture, engineering, survey, permits, utility connections — add $60,000–$130,000 to most detached DADU projects. See Garage Conversion ADU Seattle Eastside for garage conversion specifics, and ADU vs DADU in King County 2026 for a type comparison.

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The Two-ADU Strategy: Maximize Your Lot

HB 1337’s most underused provision is the two-ADU allowance. Most Eastside homeowners who are thinking about an ADU are thinking about one unit. But the law allows two.

The two-ADU configuration that pencils best on most Eastside lots: one garage conversion ADU (attached or detached) for immediate rental income, and one detached DADU for a family member or long-term tenant. The garage conversion typically costs $80,000–$180,000 and generates $1,800–$2,500/month in rental income. The DADU costs $350,000–$550,000 and generates $2,200–$3,200/month.

At $2,500 + $3,000/month combined rental income ($66,000/year), an Eastside homeowner can service approximately $1,000,000 in construction debt at 7% interest — which is roughly what a well-designed two-ADU project costs on a good Eastside lot.

This strategy requires design from the start: the garage conversion and DADU need to be designed together for parking, utility allocation, and site circulation. Retrofitting the second unit after the first is built typically adds $20,000–$50,000 in duplicated utility and site costs.

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Frequently Asked Questions

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Work With Piper Cole Architects on Your Eastside ADU

David Meade, AIA, NCARB leads Piper Cole Architects, a licensed AIA firm based in Kirkland. We help homeowners across Kirkland, Bellevue, Redmond, Sammamish, and the Seattle Eastside navigate the ADU process — from site feasibility through permit approval and construction administration.

The ADU regulatory landscape has changed substantially. If you’ve been curious about an ADU and assumed it was too complicated or too restricted, the rules are now meaningfully different than they were two years ago.

Start with a conversation.

Contact Piper Cole Architects

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Sources

• Washington State HB 1337 (Engrossed Second Substitute) — final bill text: app.leg.wa.gov
• RCW 36.70A.696 — ADU provisions as codified: app.leg.wa.gov
• Bellevue Ordinances 6851 and 6852 — ADU reform: bellevuewa.gov/code-amendments
• City of Kirkland ADU Code Amendment Summary: kirklandwa.gov
• City of Redmond Ordinance 3186 and 3220: redmondwa.gov
• King County Department of Local Services — ADU guidance: kingcounty.gov