ADU FAQ

An ADU (Accessory Dwelling Unit) is a secondary housing unit on a single-family residential lot. In Washington State, ADUs come in two main forms:

AADU (Attached ADU) — A unit that shares a wall with the primary home. This includes basement apartments, garage conversions attached to the house, and additions built onto the main structure.

DADU (Detached Accessory Dwelling Unit) — A completely separate structure on the same lot as the primary home — sometimes called a backyard cottage, carriage house, or granny flat.

Both types provide additional living space and can be rented out for income or used for family members, guests, or as a home office.

Costs vary significantly based on type, size, finishes, and site conditions. Here are general ballparks for the Puget Sound region:

DADU (Detached ADU / Backyard Cottage): $180,000–$350,000+ for a fully permitted, site-built unit. Prefab or modular DADUs can come in lower but still require foundation and utility connections.

Garage or Basement Conversion (AADU): $100,000–$200,000, depending on the condition of the existing space and the extent of renovation needed.

These figures include design, permitting, and construction but can vary widely based on your specific lot and project scope. The best way to get an accurate number is to request quotes from local builders.

Yes. All ADUs in Washington State require a building permit. In Seattle, permits are issued through the Seattle Department of Construction & Inspections (SDCI). Other cities in the Puget Sound — including Bellevue, Tacoma, Kirkland, and Renton — each have their own permitting departments and may have slightly different requirements.

Permitting covers zoning approval, building plans review, and inspections during construction. Most licensed ADU builders will handle the permitting process on your behalf — it's a good question to ask when interviewing contractors.

Plan for roughly 10–18 months total from design kickoff to move-in, broken down roughly as:

Design & engineering: 1–3 months
Permitting: 3–6 months (varies significantly by city and permit type)
Construction: 4–8 months

Timelines can vary based on your city's permitting backlog, contractor availability, and project complexity. Starting early and hiring an experienced builder who knows the local permitting process is the best way to avoid delays.

Long-term rentals: Yes. Renting your ADU on a long-term basis (month-to-month or annual lease) is permitted across Washington State. Many homeowners build ADUs specifically to generate rental income to offset their mortgage or build equity.

Short-term rentals (Airbnb, VRBO): This depends on your city's local regulations. Seattle, for example, has specific rules about short-term rentals and requires a license. Check with your city's planning department or a local attorney before listing on short-term rental platforms.

No. Under House Bill 1337, signed into Washington law in 2023, the owner-occupancy requirement for ADUs was eliminated statewide. You are no longer required to live in either the primary home or the ADU to build or rent one out.

This was a significant change from previous rules, which required the property owner to occupy one of the units. The new law opened ADU construction to a much wider range of property owners and investors.

AADU (Attached ADU) is connected to the main house — examples include a converted basement with a separate entrance, a garage apartment attached to the home, or an addition built onto an existing structure. AADUs tend to cost less because they share walls and sometimes utilities with the main home.

DADU (Detached ADU) is a fully separate structure on the same lot — a standalone backyard cottage, carriage house, or prefab unit. DADUs offer more privacy for tenants and typically command higher rental rates, but they cost more to build since everything (foundation, framing, roof, utilities) is built from scratch.

Both count toward your property's housing potential under Washington's updated zoning laws.

Choosing the right builder is the most important decision in your ADU project. Here's what to look for:

ADU-specific experience: ADUs have unique permitting and design challenges. Look for builders who have completed multiple ADU or DADU projects — not just general contractors who occasionally build them.

Washington State contractor license: Verify the builder is licensed and bonded through the WA Department of Labor & Industries. You can look up any contractor at L&I's online lookup.

Transparent pricing: A good builder will give you a detailed written estimate and explain what's included. Be cautious of quotes that are vague or dramatically below market.

References and reviews: Ask for references from past ADU clients. Check Google reviews and ask how the builder handled problems or delays on past projects.

You can browse verified ADU builders in our directory and request free quotes from multiple contractors at once.

These are two landmark Washington State housing bills passed in 2023 that significantly expanded homeowners' rights to build additional housing on their property.

House Bill 1110 requires cities with populations over 25,000 to allow middle housing — including duplexes, triplexes, and in many cases fourplexes — in areas that were previously zoned exclusively for single-family homes. This dramatically increases the density potential of residential lots across the Puget Sound.

House Bill 1337 specifically addressed ADUs. It eliminated owner-occupancy requirements, removed parking mandates tied to ADUs, and streamlined the permitting process for accessory dwelling units across Washington State. Cities can no longer prohibit ADUs on lots that otherwise qualify.

Together, these laws represent the most significant shift in Washington's residential zoning in decades and create real opportunities for homeowners to build equity and generate rental income.

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