Go Far®The short answer
In the Seattle–Snohomish County market, most attached ADU projects land between $100,000 and $250,000, depending on where the unit starts from. Converting existing space — a basement or an attached garage that already has a roof, walls, and a slab — sits at the low end of that band. Building an attached addition with new foundation and framing sits at the top, and a fully detached backyard cottage runs higher still (see our DADU cost guide for that math). Garage conversions are their own case: roughly $30,000–$60,000 to become finished living space, or $60,000–$100,000 to become a legal, self-contained ADU.
What actually drives the number
- The starting structure. Every wall, slab, and roof plane you can keep is money you do not spend twice. A dry, full-height basement is the cheapest ADU in Washington; a sloped crawl-space foundation is not.
- The kitchen and the bath. An ADU is legally a complete dwelling, so it needs both — and they are the two most expensive rooms per square foot in any project. Plumbing routes matter more than finishes here.
- Egress, light, and ceiling height. Basements often need a new egress window or door, and sometimes floor lowering to hit legal ceiling heights. These items are invisible in the after photos and very visible in the budget.
- Separate systems. Sound insulation, a dedicated heating solution (usually a ductless heat pump), and sometimes an electrical panel upgrade. Older homes in Lynnwood, Edmonds, and Everett frequently need the panel work regardless — an ADU just moves it up the calendar.
- Soft costs. Design, engineering, and permits typically add about 10–15% on top of construction. A conversion permit is cheaper and faster than a new-structure permit — another reason conversions pencil first.
Washington law is on your side
Since the state’s 2023 ADU reform (HB 1337) took effect, cities in urban growth areas must allow at least two accessory dwelling units per residential lot, cannot require the owner to live on site, and cannot demand extra parking near frequent transit. Every city we serve has now updated its code to comply. In practice this means the question is rarely whether you can add an ADU in Lynnwood, Seattle, or the Eastside — it is which kind your lot and budget favor. Our ADU & DADU rules guide covers the details city by city.
Attached vs. detached: how owners choose
If the goal is rental income at the lowest entry price, converting existing space wins — the payback window on a basement ADU renting near Seattle-area rates is often shorter than anything else in remodeling. If the goal is multigenerational living with real separation, or maximum long-term property value, the detached cottage earns its premium. Many of our clients ultimately do both, in stages — the 2023 law explicitly allows it on one lot.
Get a real number for your house
Go Far Construction builds attached ADUs, DADUs, and garage conversions across Lynnwood, Seattle, and Snohomish & King County. A free on-site feasibility visit covers your zoning, utility routing, and an itemized budget range — before you commit to drawings.