ADUs in Clark County: What They Cost and What's Now Allowed (2026)
Washington just made it far easier to add an accessory dwelling unit, and most Clark County lots can now have up to two with no owner-occupancy rule. Here is what a garage conversion, in-law suite, or detached ADU actually costs, and how to decide which one fits.
An accessory dwelling unit, or ADU, is a second smaller home on your property. It might be a converted garage or basement, a suite attached to your house, or a standalone cottage in the backyard. People build them for aging parents, adult kids, a home office, or rental income, and Washington just made them much easier to add.
If you have looked into an ADU before and walked away, the rules you ran into have likely changed. Here is what an ADU costs in Clark County in 2026, what the state now allows, and how to figure out which type actually fits your lot and your goal.
What changed: Washington's ADU law
Washington passed House Bill 1337, which requires cities and counties to allow ADUs on most lots that already permit a single-family home. The headline points for homeowners:
- **Up to two ADUs** are allowed on most single-family lots inside urban growth areas.
- **No owner-occupancy requirement.** You no longer have to live on the property to have an ADU, which is what makes them work as rental income, not just family housing.
- **No tiny size caps.** Cities cannot limit an ADU to less than 1,000 square feet. Vancouver caps an ADU at 1,000 square feet.
The exact rules still depend on your specific address and jurisdiction, and they continue to phase in. The state sets the floor through HB 1337, Clark County governs unincorporated areas, and each city (Vancouver, Camas, Washougal, Ridgefield, and Battle Ground) sets its own details on size, setbacks, parking, and design. We have linked the official sources for the state, the county, and the cities at the bottom of this post so you can read the rules yourself, and confirming exactly what applies to your lot is the real first step on any ADU.
The direction, though, is clear: ADUs went from a special-case project to one of the most homeowner-friendly moves on the board.
The three types, and what they cost
ADUs come in three shapes, and the cost gap between them is large because the amount you are building is so different. These are honest, realistic ranges for an average-size unit; finish level and size move them.
Garage or basement conversion: from about $45,000
The most affordable path, because the walls, roof, and foundation already exist. You are finishing and outfitting the space: framing, insulation, a full kitchen and bath, heating, electrical, and the egress and light a living space needs. A typical conversion runs roughly $45,000 to $105,000 and up.
Attached ADU / mother-in-law suite: from about $80,000
The middle path. An addition or reworked wing with its own kitchen or kitchenette, full bath, and private entrance, tied into the home's structure and systems. More square footage than a conversion, more independence than a spare room. A typical attached suite runs roughly $80,000 to $155,000 and up.
Detached ADU: from about $140,000
A standalone home in the backyard, built from the ground up: its own foundation, framing, roof, kitchen, bath, and utilities. The most private and most rentable type, and the biggest investment. A typical detached unit runs roughly $140,000 to $270,000 and up.
These are planning ranges. The real number comes from a walkthrough and a feasibility check for your specific lot, where utilities, slope, access, and your finish choices set the actual cost.
Is an ADU worth it? The ROI question
An ADU is one of the few home projects that can pay you back two ways: rental income while you own the home, and added value when you sell. With owner-occupancy no longer required, a unit can be rented long-term or used for family today and income later. That flexibility is the point.
It is also where we think a contractor should earn their keep. The right move is not always the biggest unit. It is the one whose cost, rent, and resale math actually works for your lot and your goal. We would rather help you size that correctly than sell you the largest build.
How we approach an ADU
We handle an ADU as one accountable engagement: a feasibility and zoning check for your address, design matched to your budget and goal, permitting through Clark County or the City of Vancouver, and the build itself, with one point of contact start to finish.
And we think past the project. In the 360 Method, an ADU is an Upgrade that should grow your home's value, and the stage after it is keeping that investment maintained and documented so it holds. We are not here to drop a unit in your yard and leave. We are your partner in where the property is going.
See the three ADU types side by side and estimate yours. Explore ADUs & estimate cost