Frequently Asked Questions

An ADU (Accessory Dwelling Unit) is a smaller, self-contained residential unit on the same lot as a primary home. A DADU (Detached Accessory Dwelling Unit) is an ADU that is a separate structure from the main house, often called a backyard cottage, mother-in-law suite, or laneway house.

ADUs can also be attached to the main home or created by converting existing space like a basement or garage. They include their own kitchen, bathroom, and sleeping area.

In the Seattle metro area, a new DADU typically costs between $180,000 and $350,000+, depending on size, finishes, site conditions, and complexity. Garage conversions and basement ADUs can sometimes come in lower, in the $100,000 to $200,000 range.

Key cost factors include foundation work, utility connections, permitting fees, and design. Getting quotes from multiple builders is the best way to understand pricing for your specific property.

Yes. Building an ADU in Washington State requires permits from your local jurisdiction. In Seattle, you'll need to go through the Seattle Department of Construction and Inspections (SDCI). Other cities like Bellevue, Tacoma, and Kirkland have their own permitting processes.

Most reputable builders will handle the permitting process for you as part of their scope of work. Make sure to ask about this when getting quotes.

From start to finish, most ADU projects take 10 to 18 months. This includes design (2-4 months), permitting (2-6 months depending on the city), and construction (4-8 months).

Prefab and modular ADU builders can sometimes compress the construction timeline significantly, with some offering move-in ready units in as few as 3-4 months after permits are approved.

Yes. In most Washington cities, you can rent out your ADU as a long-term rental. Short-term rental rules (like Airbnb) vary by jurisdiction. Seattle, for example, allows short-term rentals in ADUs under certain conditions, while other cities may have restrictions.

Rental income from an ADU can significantly offset your mortgage or provide a steady income stream. Many homeowners report earning $1,500 to $2,500+ per month from a well-located ADU.

Not anymore in most cases. Washington's HB 1337 (passed in 2023) removed owner-occupancy requirements for ADUs statewide. This means you do not need to live in either the primary home or the ADU to build or rent one out.

This was a major change that opened up ADU construction for investors and property owners who don't live on-site.

An AADU (Attached Accessory Dwelling Unit) is built within or attached to the primary home. This includes basement conversions, garage conversions, or additions built onto the existing structure.

A DADU (Detached Accessory Dwelling Unit) is a completely separate building on the same lot, like a backyard cottage or carriage house. DADUs typically cost more but offer more privacy and flexibility for both the homeowner and tenants.

When evaluating ADU builders, look for:

Experience: Have they completed ADUs similar to what you want? Ask for photos and references from past projects.

Licensing: Verify they have a valid Washington State contractor's license and are bonded and insured.

Transparency: A good builder will provide a detailed estimate, a clear timeline, and communicate regularly throughout the project.

Reviews: Check Google reviews and ask for homeowner references. Our directory shows ratings for each builder to help you compare.

HB 1110 is Washington's middle housing bill, passed in 2023. It requires cities to allow duplexes on all residential lots and fourplexes on lots near transit. This dramatically increased housing density options statewide.

HB 1337 is the ADU reform bill, also passed in 2023. It requires cities to allow at least two ADUs per lot, eliminates owner-occupancy requirements, reduces parking mandates, and prevents cities from imposing excessive impact fees on ADUs.

Together, these bills have made it significantly easier and more financially attractive to build ADUs in Washington.

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