Washington ADU Laws 2026: What Eastside Homeowners Need to Know

*By David Meade, AIA, NCARB — Licensed Architect, Piper Cole Architects*

If you own a home on the Eastside and you’ve been thinking about adding a backyard cottage, an in-law suite, or a rental unit, 2026 is probably the best time to move forward. Two major Washington state laws have reshaped what you can build — and for most homeowners in Kirkland, Bellevue, Redmond, Sammamish, Issaquah, Mercer Island, and Woodinville, the answer is: more than you think.

This guide breaks down the current rules city by city. I’ll cover both laws, what each one means for your property, and exactly what to expect when you start the permit process.

The Two Laws That Changed Everything

HB 1337: Two ADUs Per Urban Lot (Effective Since 2023, Fully in Force 2024)

House Bill 1337 is the most sweeping ADU reform Washington has ever seen. Passed in 2023 and requiring city compliance by July 2024, it applies to every city planning under the Growth Management Act with a population over 25,000. Here is what it requires:

• Two ADUs allowed per residential lot. You can build one attached ADU (basement conversion, in-law suite, above-garage apartment) and one detached ADU (backyard cottage, garden house) — simultaneously.
• No owner-occupancy requirement. You are no longer required to live on the property to rent either unit. This matters enormously for investors and homeowners who want to move while keeping the property.
• Minimum 1,000 sq ft per ADU. Cities cannot cap ADU size below 1,000 sq ft (or 60% of the primary dwelling’s floor area, whichever is less). Some cities allow more.
• No excessive impact fees or parking mandates. Cities cannot use fees or parking minimums as backdoor barriers.

The practical result: a single-family lot across the Eastside can now legally house three independent living units — the main home plus two ADUs. For homeowners carrying a large mortgage, that is a significant income opportunity. For families who want a parent or adult child nearby, it changes what is possible without selling.

Learn more about the difference between an attached and detached unit in our ADU vs DADU in King County 2026 guide.

HB 1345: Rural ADUs Now Allowed (Effective June 11, 2026)

House Bill 1345, signed by Governor Bob Ferguson on March 27, 2026 and effective June 11, 2026, extends ADU rights to rural properties outside Urban Growth Areas. Before this bill, most rural-zoned parcels in Washington had no legal path to building an ADU.

Key provisions of HB 1345:

• One detached ADU allowed per rural parcel in counties that opt in under the Growth Management Act.
• Maximum size: 1,296 sq ft — slightly larger than the urban minimum.
• Must be sited within 150 feet of the primary dwelling.
• Shared driveway required. No separate access road.
• Water and septic compliance required. No ADU is permitted where water supply is closed to further appropriation.
• Short-term rentals prohibited for rural ADUs created under this law.
• County participation is voluntary. Each county must opt in. King County’s adoption status for unincorporated areas — including rural Woodinville and rural Sammamish fringe parcels — should be confirmed with the King County Department of Local Services.

If your property is in an unincorporated area or near the rural fringe, HB 1345 may have opened a door that was previously shut. This is especially relevant for large acreage parcels in eastern Woodinville and rural Sammamish.

Eastside City-by-City ADU Rules: 2026

General state minimums apply everywhere in the Urban Growth Area. But cities add their own overlay rules on top — setbacks, height limits, design standards, and GFA calculations. Here is where each Eastside city stands today.

*Note: Sizes reflect minimum required by state law or adopted local code as of June 2026. Always verify current setback, height, and lot coverage rules with each city’s planning department before designing.*

City-by-City Deep Dive

Kirkland

Kirkland moved quickly after HB 1337 passed. It is classified as a Tier A city under HB 1110, meaning it must also allow middle housing types like duplexes and triplexes. For ADUs specifically, Kirkland allows two per residential lot with no owner-occupancy requirement and no minimum parking requirement if the property is within half a mile of a frequent transit stop.

One standout feature: Kirkland has an active pre-approved detached ADU plans program. Using a pre-approved plan can cut permit review time significantly — a major advantage when construction costs are time-sensitive.

Bellevue

Bellevue adopted its middle-housing code update on June 24, 2025, bringing it into full HB 1337 compliance. Two ADUs are now allowed per lot in all residential zones. Setbacks in most zones are 5 feet rear and side for detached ADUs. Height maximum is 24 feet for pitched-roof structures, 18 feet for flat roofs.

Bellevue is actively developing a pre-approved DADU plans pilot program in 2026. When launched, this will offer homeowners a faster track through permitting. Permit review currently runs 4–8 weeks, faster than Seattle’s 8–14 week average.

See our full Bellevue ADU rules 2026 breakdown for setback diagrams and zone-specific details.

Redmond

Redmond updated its code in May 2025 via Ordinance 3186, aligning with HB 1337. Notably, Redmond allows up to 1,500 sq ft for an ADU — well above the state minimum. Two ADUs are permitted per lot. Owner occupancy is not required for long-term rentals, but if you use either unit as a short-term rental (lease under 12 months), the owner must reside on-site.

Detached ADUs in Redmond must match the facade, roof pitch, siding, and windows of the primary home — a design standard worth factoring into your budget early.

Sammamish

Sammamish is HB 1337 compliant, allowing two ADUs per lot. The detached ADU cap is 1,000 sq ft. For attached ADUs, the limit is 1,000 sq ft or 50% of the primary home’s square footage, whichever is larger — so a larger home unlocks a larger attached unit. ADUs do not count toward FAR calculations, which is a meaningful benefit on smaller lots.

Sammamish is still refining its permanent ADU code updates; interim regulations are in effect. Confirm current setback requirements with the Planning Department before finalizing a design.

Issaquah

Issaquah has adopted HB 1337-compliant regulations allowing two ADUs per residential lot. Each individual ADU is capped at 1,000 sq ft, excluding non-living areas such as garage space or storage sheds.

One important local overlay: if your property is in the Issaquah Highlands development, the Issaquah Highlands Community Association’s Architectural Review Committee (ARC) has design standards that sit on top of city code. Any ADU design must pass ARC review before permit submission. Factor this into your timeline — ARC reviews are separate from the city process and can add 4–6 weeks.

Mercer Island

Mercer Island adopted interim regulations in February–March 2025 to comply with HB 1337 and is developing permanent code in 2026. Two ADUs are allowed per lot. Owner occupancy is not required for standard residential rentals (only for short-term rentals per state law).

A key local rule: Mercer Island applies a Gross Floor Area cap — total square footage of ADUs plus other accessory structures cannot exceed 25% of the total allowed GFA for the parcel. On a larger lot this is rarely binding, but on smaller lots in Mercer Island’s dense neighborhoods it can limit your effective ADU size below the 1,000 sq ft maximum. A licensed architect can run this calculation for your specific parcel before you commit to a design.

Our ADU design services page covers how we approach this kind of feasibility work.

Woodinville

Woodinville’s incorporated city limits fall within the Urban Growth Area and are subject to HB 1337 — two ADUs per lot, no owner-occupancy requirement, 1,000 sq ft minimum size. Standard Eastside rules apply.

The bigger opportunity in 2026 is for homeowners on rural-zoned or large-acreage parcels in the Woodinville area, which are governed by unincorporated King County rather than the city. HB 1345 now authorizes King County to allow one detached ADU on these rural parcels — up to 1,296 sq ft — if King County opts in. Woodinville is one of the most rural-adjacent communities on the Eastside, making this law particularly relevant here.

If you own acreage in east Woodinville, Cottage Lake, or other rural-zoned King County parcels near Woodinville, now is the time to check with King County’s Department of Local Services about their HB 1345 adoption timeline.

What This Means If You Are Thinking About Building

Whether you are adding a rental unit, building a space for a parent, or planning a multi-generational compound, a few things are true across every Eastside city in 2026:

1. You can build two units on most residential lots. The state has removed the legal obstacles that blocked this for decades.
2. You do not need to live on the property to rent either unit. This opens investment strategies that were previously off-limits.
3. Size, setbacks, and design standards still matter. State law set the floor — cities still have room to regulate above it.
4. Rural property owners have a new option. HB 1345 is brand new. If you have acreage, get ahead of the crowd.

The question for most homeowners is no longer “can I build an ADU?” — it is “what is the best ADU design for my specific lot, and how do I get through permits efficiently?”

That is exactly what we do at Piper Cole Architects. We have worked on ADU projects across Kirkland, Bellevue, Redmond, Sammamish, Issaquah, Mercer Island, and Woodinville. We know the local codes, the planning staff, and the design standards that each jurisdiction enforces.

If you are also curious how middle housing legislation is pushing density further — triplexes and fourplexes — see our overview of triplex and fourplex design under HB 1110.

And if you are wondering what architectural services actually cost before committing, read our honest breakdown of how much does an architect cost in Seattle.

Frequently Asked Questions

How many ADUs can I build on my property in Washington state?

In cities over 25,000 people that plan under the Growth Management Act — which includes all Eastside cities like Kirkland, Bellevue, Redmond, Sammamish, Issaquah, Mercer Island, and Woodinville — you can build two ADUs per residential lot under HB 1337. One can be attached (basement suite, garage conversion) and one can be detached (backyard cottage). This is in addition to the primary home.

Does HB 1337 apply to all Eastside cities?

Yes. All fully planning cities with populations over 25,000 were required to comply with HB 1337 by July 2024. Every Eastside city covered in this guide — Kirkland, Bellevue, Redmond, Sammamish, Issaquah, Mercer Island, and Woodinville — has adopted compliant regulations. Individual cities may have additional design standards, setback requirements, or size limits on top of the state minimums, so city-specific rules still matter.

What changed with HB 1345 for rural properties in Washington?

HB 1345, effective June 11, 2026, allows counties to permit one detached ADU on rural-zoned parcels outside Urban Growth Boundaries. The ADU can be up to 1,296 sq ft, must be sited within 150 feet of the primary home, must share a driveway, and cannot be used as a short-term rental. County participation is voluntary — each county must opt in. If you have a rural-zoned or large-acreage property near Woodinville or on the rural Sammamish fringe, contact King County’s Department of Local Services to confirm their adoption status.

Do I need a permit to build an ADU in King County?

Yes. All ADUs — whether attached or detached, new construction or conversion — require a building permit in King County and all Eastside cities. Permits are pulled through each city’s building department (or King County’s Department of Local Services for unincorporated areas). Working with a licensed architect streamlines permit preparation and reduces the risk of revisions that add time and cost.

What is the maximum size for an ADU in Washington state?

State law sets a minimum allowed size of 1,000 sq ft — cities cannot cap ADUs below this threshold. Some cities allow more. Redmond, for example, allows up to 1,500 sq ft. For rural ADUs under HB 1345, the cap is 1,296 sq ft. Attached ADUs in Sammamish can exceed 1,000 sq ft if the primary home is large enough (50% of the primary dwelling’s floor area applies). The actual maximum on your property depends on your city’s code, your lot size, setbacks, and lot coverage limits.