Cost guide · Updated for 2026
ADU cost in Clark County: $80,000 to $300,000
Washington rewrote its ADU laws in the homeowner's favor. Here is what a backyard cottage, garage conversion or basement unit costs to build in Vancouver and Clark County, and the size, parking and occupancy rules that now apply. Every figure names its source.
- 2 per lotADUs allowed under HB 1337
- 1,000 SFVancouver max ADU size
- NoOwner-occupancy requirement
- FreeWritten estimates
Quick answer: Published 2025 industry guides put detached new ADUs at $140,000 to $300,000, attached ADUs at $120,000 to $250,000, garage conversions at $80,000 to $175,000 and basement conversions at $75,000 to $220,000, plus $5,000 to $25,000 for permits and utility connections. Vancouver allows ADUs up to 1,000 square feet, two per lot, with no owner-occupancy requirement. Add 8.9 percent sales tax on the contract in the City of Vancouver.
What each type of ADU costs
| ADU type | Published range | Per square foot | Source |
|---|---|---|---|
| Detached new build, 400 to 800 SF | $140,000 to $300,000 | $350 to $400/SF | The House Plan Company / 247pro ADU guides, 2025 |
| Attached addition ADU | $120,000 to $250,000 | $300 to $350/SF | The House Plan Company / 247pro ADU guides, 2025 |
| Garage conversion | $80,000 to $175,000 | $250 to $300/SF | The House Plan Company / 247pro ADU guides, 2025 |
| Basement conversion | $75,000 to $220,000 | $190 to $250/SF | The House Plan Company / 247pro ADU guides, 2025 |
| Permits and utility connections | $5,000 to $25,000 additional | The House Plan Company / 247pro ADU guides, 2025 |
Where these numbers come from. These are national ranges from 2025 ADU industry cost guides; Portland-metro labor tends to sit in the middle to upper part of national ranges. Conversions are cheaper because the shell already exists. In the City of Vancouver, Washington's retail sales tax (8.9 percent, DOR rate lookup, October 2026) applies to the full construction contract on top of these figures.
The rules changed in your favor: HB 1337
Washington's 2023 ADU law (HB 1337, codified at RCW 36.70A.680) forced cities and counties inside urban growth areas to drop the rules that used to make ADUs impractical. Per the Department of Commerce's guidance, jurisdictions may no longer require owner-occupancy (except for short-term rentals), must allow two ADUs per lot, cannot cap size below 1,000 square feet, must allow 24 feet of height, cannot demand parking near major transit stops (and at most one space elsewhere on small lots), cannot charge impact fees above half the principal unit's, and must allow ADUs to be sold separately as condominiums.
Vancouver and Clark County specifics
- City of Vancouver: ADUs up to 1,000 square feet, two units per lot, no owner-occupancy requirement, and no more "visually subordinate" design rule, per the city's ADU page. From July 31, 2026 the city's updated code allows more residential structures per lot, up to two of them ADUs. One practical catch: an ADU application cannot eliminate the main home's required parking space.
- Unincorporated Clark County: urban lots allow 1,000 square feet, rising to 1,500 square feet on lots of 20,000 square feet or more; a basement ADU may use the full basement even beyond those caps so long as the ADU stays smaller than the main home; two ADUs may total 2,000 square feet combined (county ADU handout, June 2024).
- Permits: an ADU is a full residential permit with plan review: ePlans in the City of Vancouver, CC LMS in the county, electrical through WA L&I outside city limits. See the permit guide for the jurisdiction-by-jurisdiction walkthrough.
Why ADUs pencil here
Clark County keeps absorbing people from the Oregon side: moves from the Portland tri-county area jumped from under 8,000 in 2019 to nearly 14,000 in 2022 (Oregon Office of Economic Analysis, "Moving Across the River," 2024), and Zillow puts the typical Vancouver home around $502,000 (August 2026). When a whole second house is out of reach, a 1,000 square foot unit on land you already own is the affordable way to add housing for a parent, an adult child or a tenant. And because Vancouver dropped owner-occupancy, the unit remains a rental asset even if you later move.
How to use these numbers
- Start with what you have. An unused garage or full-height basement cuts the budget nearly in half versus new construction. Our ADU construction service covers all four types.
- Check utilities early. Sewer, water and panel capacity are the usual five-figure surprises; we scope them at the first site visit.
- Design to the cap. At 1,000 square feet you can fit two bedrooms; going smaller saves less than people expect because kitchens and baths are the expensive rooms.
- Budget tax and fees. 8.9 percent sales tax in the City of Vancouver plus $5,000 to $25,000 in permits and connections per the published guides.
ADU questions
What does an ADU cost to build in Clark County in 2026?
Industry cost guides published in 2025 put a detached new ADU of 400 to 800 square feet at $140,000 to $300,000 (roughly $350 to $400 per square foot), an attached ADU at $120,000 to $250,000, a garage conversion at $80,000 to $175,000 and a basement conversion at $75,000 to $220,000, with permits and utility connections adding $5,000 to $25,000. In the City of Vancouver, add 8.9 percent sales tax on the full contract. A written estimate for your lot is the real number.
How big can an ADU be in Vancouver, WA?
Up to 1,000 square feet in the City of Vancouver, per the city's current ADU rules. Unincorporated Clark County allows 1,000 square feet on standard urban lots, 1,500 square feet on lots of 20,000 square feet or more, and up to 2,000 square feet combined when a lot has two ADUs, per the county's June 2024 handout.
Do I have to live on the property to rent out an ADU?
No. Washington's HB 1337 bars cities inside urban growth areas from requiring owner-occupancy except for short-term-rental situations, and the City of Vancouver has removed its owner-occupancy requirement. That change is what makes an ADU a genuine rental investment rather than just a guest house.
How many ADUs can I build on my lot?
Under HB 1337, cities and counties inside urban growth areas must allow two ADUs per lot, attached or detached, on any lot that meets the minimum size for the principal home. Vancouver allows two units per lot now, with further flexibility arriving July 31, 2026 under the city's updated rules.
Is a garage or basement conversion cheaper than building new?
Substantially. Published 2025 guides put garage conversions at $80,000 to $175,000 and basement conversions at $75,000 to $220,000, versus $140,000 to $300,000 for a detached new build, because the shell, slab and roof already exist. Clark County explicitly allows basement ADUs to use the full basement footprint even past the normal size cap, as long as the ADU does not exceed the primary home's size. Our basement finishing service handles the conversion work.
What does an ADU rent for in Vancouver?
We do not publish rent projections because they change block by block and year by year. What we can say: Clark County's population keeps growing, with moves from the Portland side jumping from under 8,000 in 2019 to nearly 14,000 in 2022 per the Oregon Office of Economic Analysis, and a legal, separately rentable unit on an existing lot is the cheapest way to add a dwelling in this market. Talk to a local property manager for current rents, and to us for the build number.
Free on-site estimates
Ready to price your ADU?
Tell us about your lot. The registered contractor walks it with you, checks utilities and setbacks, and gives you a written scope and price. No obligation.
- Written pricing
- Permits handled
- Warranty in writing