October 20, 2026
ADU Size Limits in Clark County and Vancouver, WA
If you've looked up ADU size limits for Clark County, you may have found two different answers. Some county pages and handouts from 2018 still show an older limit of 800 square feet or 40% of the house. That limit has since been replaced: the county's ADU rules were updated in 2024 and amended again in 2025, and the current limits are larger. Here's how the rules compare today across the City of Vancouver, urban Clark County and rural Clark County.
Last checked: October 19, 2026. ADU rules change, so confirm with the jurisdiction before you design.
The rules side by side
| City of Vancouver | Clark County, urban | Clark County, rural | |
|---|---|---|---|
| How many per lot | Up to 2 | Up to 2 | 1 |
| Size per ADU | 1,000 sq ft or less to count as an ADU | 1,000 sq ft, or 1,500 on lots of at least 20,000 sq ft | Up to 1,500 sq ft total floor area (excludes garages) |
| Combined limit | Not specified | 2,000 sq ft | Not applicable |
| Attached or detached | Attached, detached or conversion | Attached or detached | Must be attached (shares a common wall) |
City of Vancouver
The city allows up to two ADUs per lot. A unit counts as an ADU only if it's 1,000 square feet or less. A bigger unit may still be allowed, but the city doesn't treat it as an ADU, and it doesn't get the reduced impact fees. Attached ADUs, detached ADUs and conversions of existing space are all allowed. A detached ADU has to be 10 feet from other structures or meet extra fire-code requirements.
Source: City of Vancouver ADU page. Code chapter VMC 20.810, last amended in 2025. Per-unit size and density (up to two) are in VMC 20.810.040; the code does not state a combined maximum for two ADUs on one lot.
Clark County, urban
In urban unincorporated Clark County, you can build up to two ADUs per lot. Each can be up to 1,000 square feet, with 2,000 square feet combined. On lots of at least 20,000 square feet, one ADU can be up to 1,500 square feet, so a 1,500 and 500 square foot split works on those larger lots. A basement ADU can be larger, up to the size of the main house.
The county's urban ADU rules were updated in 2024 and amended again in 2025. Ord. 2025-07-01 was adopted July 15, 2025, and took effect July 25, 2025 (§30: 10 days after adoption) (Ord. 2025-07-01, PDF). The older 800 square feet or 40% limit has since been replaced.
Source: CCC 40.260.020.
Clark County, rural
Rural lots are different. The county allows one rural ADU per lot, and it must be attached to the house, meaning it shares a common wall. The 1,500 square foot limit is total floor area, including nonhabitable space but not counting garages. A basement ADU with a separate exterior entrance can use the whole basement, as long as it doesn't exceed the size of the main house (CCC 40.260.022(C)(6)(b)). The county lists four ways to create a rural ADU: an internal conversion, an addition to the house or an attached garage, converting an existing attached garage, or building it as part of a new house.
The county says rural ADUs provide housing "while maintaining rural community character and ensuring the conservation, enhancement, and protection of resource lands."
My understanding is that the county wants to keep people from building a detached ADU, subdividing the property and selling the ADU separately.
Source: Clark County rural ADU page; code section CCC 40.260.022.
A few related rules worth knowing
- You don't have to live on the property. Neither the City of Vancouver nor Clark County requires the owner to live on site. Clark County removed the owner occupancy requirement in 2018 (ADUs and cottage housing). Owner-occupancy remains absent from the live urban and rural ADU code sections (CCC 40.260.020 and CCC 40.260.022, including Ord. 2025-07-01).
- Impact fees are reduced. Vancouver charges ADU impact fees at 50% of the main home's fees. Clark County waives 75% of certain impact fees, figured at the multifamily rate: transportation, school and park fees for urban ADUs, and school and transportation fees for rural ADUs. Sources: Clark County urban ADU page, rural ADU page; rural transportation impact fee at the multifamily rate: CCC 40.260.022(C)(8).
Before you design
Confirm whether your lot is in the City of Vancouver, urban Clark County or rural Clark County (Clark County permit guide), since the limits differ for each. Then check your lot size if you're hoping for a 1,500 square foot unit in urban Clark County. For more on the Vancouver permit process, read ADU Permits in Vancouver, WA: 2 Surprises, and try the ADU calculator to check your numbers.
My work covers the permit itself: I prepare and submit the application and track it through the jurisdiction's decision. You get a written scope and a fixed quote, with correction responses handled within that scope. Once construction starts, each trade pulls its own permits, like mechanical, electrical and plumbing, and that part is outside my scope.
FAQ
How big can an ADU be in Clark County? In urban areas, up to 1,000 square feet each, with 2,000 combined, and one ADU can be up to 1,500 square feet on lots of at least 20,000 square feet. In rural areas, one attached ADU up to 1,500 square feet of total floor area, not counting garages.
How many ADUs can I build in Vancouver, WA? Up to two, each 1,000 square feet or less.
Can a rural ADU in Clark County be detached? No. It must be attached to the house.
Is the 800 square foot limit still in effect? No. It came from the county's older rules and has since been replaced.
Not sure which rules apply to your lot? Get my permit quote