What changed about ADUs in Seattle in 2025?

Two Washington state laws — House Bill 1110 and House Bill 1337 — took effect in 2025 and significantly changed Seattle's ADU rules. Owner-occupancy requirements were eliminated. Seattle's residential zones now allow up to multiple units per lot, including ADUs. Height limits for DADUs increased to 32 feet in some zones. Seattle also passed its own code updates in July 2025 to implement these state requirements. The result: ADUs and DADUs are now much more feasible on most Seattle residential lots than they were before.

If a property I'm buying has an existing ADU, what should I check?

Check whether the ADU has a permit. An unpermitted ADU creates legal exposure for the new owner — the city can require removal or bring it up to code. Verify whether the ADU is legally configured for rental (some properties had owner-occupancy requirements in past permits). Confirm the sewer and utility connections are properly set up. Check whether the ADU affects your lender's review — some lenders count rental income from an ADU in qualifying calculations, but only with documentation of a current lease.

Can I count ADU rental income when qualifying for a mortgage?

It depends on the loan type and whether you have documented rental history. Fannie Mae allows rental income from an ADU to be counted toward qualifying income if you can document it — typically with a lease agreement and sometimes a history of rental receipts. FHA has its own rules. An unpermitted ADU or one without documentation typically cannot be used for income qualifying. Ask your lender specifically before counting on ADU income in your qualification.

What does it cost to build a DADU in Seattle?

DADU construction costs in Seattle vary significantly based on size, configuration (new build vs. garage conversion), site conditions, and finishes. A garage conversion is typically less expensive than a new detached structure. Verify current cost ranges with contractors and permit consultants — figures change with material costs and labor markets. Budget also for permit fees, utility connection fees (which can be significant), and design costs.

ADUs and DADUs in Seattle: What Buyers and Homeowners Need to Know After 2025

Short answer

An accessory dwelling unit (ADU) is a secondary living space on a residential property. In Seattle, "ADU" typically refers to an attached unit; "DADU" (detached accessory dwelling unit) refers to a separate structure in the backyard, sometimes called a backyard cottage.

Seattle's ADU rules changed significantly in 2025 due to state legislation. Owner-occupancy requirements are gone. More units per lot are allowed. DADUs can be taller. These changes make ADUs more broadly feasible — and make ADU-related questions more relevant for buyers evaluating properties.

Whether you're buying a home that already has an ADU or considering building one, the key questions are the same: is it permitted, is it compliant, and does it work for your specific situation?

What changed in 2025

Two Washington state laws drove significant changes to Seattle's residential zoning:

House Bill 1337 (effective 2024, with Seattle compliance by 2025): Required cities to allow ADUs on all lots that allow single-family homes, eliminate owner-occupancy requirements, and reduce barriers to ADU construction.

House Bill 1110 (effective 2025): Required cities in King County's urban growth areas to allow increased density in residential zones — effectively allowing more homes per lot than traditional single-family zoning permitted.

Seattle's implementation (July 2025): Seattle passed its own code updates to comply with state law. Key changes include:

No owner-occupancy requirement: you don't need to live on the property to have an ADU or rent it out
Multiple units allowed per lot in Neighborhood Residential zones
DADU height limit increased to 32 feet in some zones (up from 18 feet)
Alley-adjacent lots can build DADUs to the alley line with no setback

The practical result: ADUs are now more broadly feasible for homeowners and more attractive to buyers who are thinking about building one, renting one out, or housing a family member.

ADU types: what you might find

Attached ADU (AADU): A self-contained living space within or attached to the main house — a basement apartment, a converted garage attached to the house, or an addition with separate access. The unit has its own entrance, kitchen, and bathroom.

Detached ADU (DADU): A separate structure in the backyard — a standalone cottage, a converted garage, or a purpose-built unit. In Seattle, DADUs are commonly called "backyard cottages."

Junior ADU (JADU): A unit created within the existing footprint of the primary house, typically in a bedroom with a shared or private entrance and at minimum a kitchenette. JADUs are smaller and have different permit requirements than full ADUs.

If the property you're buying has an existing ADU

An ADU on a property you're buying can be a genuine asset — rental income potential, multigenerational living space, or a home office — but only if it's properly permitted and configured.

Check the permit status

This is the most important step. An unpermitted ADU creates legal exposure for the buyer:

The city can require it to be brought up to code or removed
An unpermitted unit cannot be legally rented
If damage or injury occurs in an unpermitted space, insurance coverage may be affected

How to check: Pull the permit history for the property address from Seattle's permit portal (Seattle Department of Construction and Inspections, sdci.seattle.gov) or the relevant city's permit system if the property is in a different municipality. Look for an ADU permit, a final inspection, and a certificate of occupancy if applicable.

An ADU with a permit and a final inspection is in a very different position than one built without permits. The permit status should appear in the listing disclosure (Form 17), but verify independently.

Verify the utility connections

An ADU needs water, sewer, and electrical connections. A legally permitted ADU will have these configured and inspected. An older or unpermitted ADU may share connections in ways that don't meet current code.

A sewer scope and inspection of the utility connections is worth including in your due diligence if the ADU is a meaningful part of the property's value or your plans. See sewer scope in Seattle for why this matters.

Understand rental restrictions

Owner-occupancy requirements have been eliminated in Seattle's residential zones as of 2025 state law. However, properties in HOA-governed communities or in unincorporated King County (which falls under county code, not Seattle code) may have different restrictions. If the property is in an HOA, review the CC&Rs for any restrictions on ADU rental or use.

Lender treatment of ADU rental income

If you plan to use future ADU rental income to help qualify for your mortgage, understand the rules before you count on it:

Fannie Mae allows rental income from an ADU with documentation (typically a current lease and market rent analysis)
An unpermitted ADU's income usually cannot be used for qualification
FHA and VA have their own rules; ask your lender specifically

How an ADU affects your offer analysis

If the ADU is permitted and in good condition, it typically adds value. The amount depends on the local market for ADU-equipped properties, the quality and size of the unit, and the rental income potential.

If the ADU is unpermitted or has compliance issues, the cost to resolve those issues should factor into your offer or your request for a price adjustment.

If you're planning to build an ADU or DADU

Whether you've just bought a property or are buying specifically to add an ADU, here's what to verify before planning.

Start with zoning and lot eligibility

Not every lot can support an ADU or DADU. Start with the City of Seattle's permit and zoning resources, or the relevant city's permit portal, to confirm what your specific lot can accommodate:

Zoning designation
Lot coverage limits
Setback requirements (five feet from most property lines in standard cases; zero setback from alleys for alley-adjacent lots)
FAR (floor-area ratio) limits, which now primarily govern unit size rather than a strict square-foot cap per unit
Height limits in your specific zone

Seattle's SDCI offers pre-permit review options that can clarify feasibility before you commit to plans.

Permit requirements

Building an ADU or DADU in Seattle requires a permit. The process includes design review, plan approval, and inspections at various stages. Permit fees and timeline vary by project complexity.

Seattle SDCI: sdci.seattle.gov

Utility connection costs

Connecting a DADU to water, sewer, and electrical service can be a significant cost — sometimes as much as the construction itself for complex site conditions. Get utility connection cost estimates before finalizing your budget. Seattle Public Utilities (for water and sewer in the city limits) can provide connection fee estimates.

Financing options

A small number of lenders offer ADU-specific renovation loans or allow future rental income projections in underwriting for buyers building ADUs immediately after purchase. Fannie Mae's HomeStyle renovation loan is one option; some lenders also offer construction-to-permanent financing for ADU projects. Ask your lender early about financing options if building is part of your plan.

What ADUs and DADUs are not

ADUs and DADUs are not a guaranteed rental income stream. Vacancy, tenant issues, maintenance, and local rental market conditions affect income. They are also not a way around HOA restrictions if the HOA governing documents prohibit ADU rental — verify this before assuming.

ADUs also don't eliminate the property's maintenance responsibilities. A DADU is a separate structure with its own roof, exterior, systems, and maintenance needs.