Guide · ADU · 14 min read
ADU Rules in King County: How State Law, City Codes and Covenants Fit Together
Whether an ADU fits on a King County lot depends on three layers of rules: Washington state law, which sets minimum rights cities must allow; your city's code, which sets size, height, setback and design standards; and any private covenants or HOA rules on the property. The building code, utilities and site conditions then decide what is practical.
Reviewed by Daniel R. Whitcomb, PE · WA PE 48217
Why are there three layers of ADU rules?
Accessory dwelling units are regulated by different bodies for different purposes. The state legislature, concerned with housing supply, sets a floor of what cities must allow. Each city writes the detailed zoning rules that apply within its boundaries, within that floor. And private agreements recorded on a property, such as subdivision covenants, can restrict what an owner builds regardless of what public rules allow.
When people say "ADUs are allowed now," they usually mean the first layer. Whether one fits on a specific lot depends on all three, plus physical constraints: room on the site, access, slope and utilities.
What does Washington state law require cities to allow?
Washington's Growth Management Act was amended to require cities and counties planning under it to allow ADUs within urban growth areas on terms set by the state. The Washington Department of Commerce publishes guidance for local governments implementing these rules. In broad terms, state law:
- Requires allowing two ADUs on lots that permit a single-family house, in attached, detached or combined forms.
- Prohibits requiring the owner to live on the property.
- Prevents capping ADU size below 1,000 square feet of gross floor area, and limits how low height limits for detached ADUs can be set.
- Limits off-street parking requirements, and removes them within a set distance of major transit stops.
- Caps impact fees on ADUs at a fraction of those charged for a new house.
- Requires allowing conversion of existing structures, such as detached garages, even when they do not meet current setbacks.
These are minimums. Cities may be more permissive, and many in King County are. The details of how each requirement is implemented vary, so the city's adopted code is the controlling document for your lot.
What do Eastside cities and Seattle still decide?
| Standard | What it controls | Why it matters |
|---|---|---|
| Setbacks | Distance from property lines | Decides where a DADU can sit in the yard |
| Lot coverage and impervious surface | Share of the lot built on or paved | Often the real limit on detached units |
| Height | Maximum DADU height above grade | Determines one or two stories |
| Size above the state minimum | Whether units larger than the floor are allowed | Affects two-bedroom layouts |
| Design standards | Entries, windows facing neighbors, roof form | Shapes privacy and appearance |
| Tree retention | Which trees can be removed | Can rule out the obvious DADU location |
| Critical areas | Slopes, streams, wetlands, shorelines | Can remove buildable area entirely |
Each city page on this site summarizes local constraints: see Bellevue, Kirkland, Seattle or the full service area list.
How are ADUs handled in unincorporated King County?
Areas outside city limits are governed by the King County Code and permitted by King County Permitting. Inside the urban growth area, state ADU requirements apply to the county as they do to cities. Rural areas outside the urban growth area have different rules, often tied to lot size, septic capacity and water availability. Addresses with a city mailing name can still be unincorporated, so confirm jurisdiction from the parcel record before planning.
Do covenants and HOA rules still apply?
Usually, yes. Recorded covenants, conditions and restrictions are private agreements among property owners, and state ADU law changes what cities can regulate, not what private agreements say. Many Eastside subdivisions, especially those built from the 1980s onward, have covenants limiting outbuildings, requiring architectural review or restricting rental use. Some covenants are old and no longer enforced; others are actively managed by an HOA.
Read your title report and any HOA documents before investing in design. If covenants appear to conflict with what you want to build, a real estate attorney can advise on whether and how they apply.
Which building code requirements shape an ADU?
Separation between units
An attached ADU is a second dwelling unit, and the residential code requires fire-resistant separation between dwelling units: rated wall and floor assemblies, sealed penetrations and often a solid-core or rated door between them. In a basement conversion, the ceiling between the units usually becomes a rated assembly, which affects ceiling height and the ductwork running through it.
Egress, light and ventilation
Every sleeping room needs an emergency escape opening of minimum size and sill height. In basements this often means cutting a larger window opening and adding a window well. Habitable rooms also need minimum ceiling heights and natural light or ventilation.
Energy and mechanical
New ADUs must meet the Washington State Energy Code, including insulation, windows, air sealing and ventilation, and must have their own heating. Ductless heat pumps are a common, efficient choice for small units.
Structure
Detached ADUs need a foundation, framing and lateral design like any small house. Conversions involve their own structural work: infilling a garage door opening, cutting new openings in foundation walls, or reinforcing floors. These are areas where an engineer is typically required.
How do utilities and fees affect an ADU?
Utilities are where ADU budgets most often grow. Sewer service may be shared with the main house or require a new side sewer, and an uphill route can require a pump. Lots on septic need approval from Public Health – Seattle & King County for the added flow. Water may be shared or require a separate meter depending on the district. Electrical service may need upgrading when a second household's heating, cooking and hot water are added.
Fees include building permits, utility connection or capacity charges set by the district, and impact fees where the city charges them, within the state's limit. The Permit Estimator gives a typical range for the building permit itself.
What should a lot feasibility check cover?
| Check | Source |
|---|---|
| Jurisdiction and zone | King County parcel viewer, city zoning map |
| Setbacks, coverage, height | City zoning code for the zone |
| Critical areas and trees | City environmental maps, site visit |
| Covenants and HOA | Title report, HOA documents |
| Sewer or septic route | Utility district maps, side sewer records, survey |
| Water and power capacity | Utility district, electrical panel |
| Access for construction | Site visit |
Next steps to check your lot for an ADU
Related questions
More about ADU rules
Can an ADU be sold separately from the main house?
Not as a simple matter of selling part of a lot. Separate ownership generally requires either a subdivision of the land, where the lot qualifies, or a condominium structure for the two units. Washington's housing laws address condominium-style ownership of ADUs, but it involves legal documents, surveys and lender considerations. Talk with a real estate attorney before planning around a sale.
Does an attached ADU need its own entrance?
An ADU must function as an independent dwelling, which in practice means its own entrance from outside or from a shared common area, not only through the main house. Some cities also regulate where the ADU entrance can face. The design typically places it at the side or rear of the house.
Will an ADU change my property tax assessment?
Yes. The King County Assessor adds the value of new construction, including ADUs and conversions that add finished living space, to the property's assessed value. The amount depends on size and quality. Rental income from the ADU is a separate matter for income tax purposes.
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