What an ADU Really Costs in Washington (2025 Reality Check)

Washington changed its ADU laws, and overnight the backyard became the most valuable square footage on a lot. Cities can no longer block most accessory dwelling units, owner-occupancy requirements have loosened, and the math suddenly works for a lot of homeowners.

So the question we hear every week is simple: what does an ADU actually cost?

Here is the honest answer.

It depends on three things

1. Size and finish. A compact studio is a different project than a two-bedroom unit with a real kitchen. Our Timber Collection is built around this — the Timber 800, 1000, and 1200 are sized so you can pick a footprint and get a real number.
2. Your site. Utility connections, setbacks, slope, and access can swing the budget more than the building itself. A flat lot with sewer at the property line is a very different job than a steep lot needing a new run.
3. Who builds it. This is where most quotes hide their surprises. A builder buying materials at retail and subbing out the framing carries every one of those markups into your price.

Why we can hold a number

Svensen mills its own oversized Douglas Fir at Washington Timber Company and engineers structures in-house. When timber prices move, that volatility hits us before it hits your quote. That is the entire reason we can offer the Timber Collection as fixed tiers instead of an open-ended bid.

The takeaway

Do not shop ADUs on square-foot price alone — shop on who controls the chain. The builder that owns its materials is the builder that can actually keep a promise on cost.

Thinking about a backyard build? Tell us about your lot and we’ll give you a straight number.