Whatcom County, WA ADU (accessory dwelling unit) Rules and Regulations (2026)

Whatcom County runs two ADU systems side by side. Eligible lots inside an Urban Growth Area got a simpler path in the county’s 2025 comprehensive plan update: up to two ADUs, no minimum lot size, no owner-occupancy rule, and a building permit without a separate land use permit. Everywhere else, the older rural framework still applies: one ADU, lot-size minimums, owner occupancy, a recorded deed restriction, and a plat check.

The standards below come from WCC 20.80.910, as amended by Ordinance 2025-060 on December 9, 2025, and the county’s ADU page. For statewide context, see our Washington ADU laws guide.

What counts as an ADU

WCC 20.97.010 defines an ADU as a smaller, independent dwelling on the same lot as a stand-alone single-family home. The county recognizes three forms:

• Internal — a converted portion of the house, such as a basement apartment.
• Attached — an addition sharing at least one wall with the house.
• Detached — a new stand-alone structure, or a converted portion of an existing stand-alone structure such as a garage or barn.

Who reviews your lot

Inside Urban Growth Areas

The UGA rules apply to lots inside designated UGAs, except those in the Lake Whatcom Watershed Overlay, General Commercial, Agriculture, Rural, and Rural Forestry districts. On those eligible lots:

• Up to two ADUs in addition to the existing or permitted house.
• Permitted use. The county says you can apply directly for a building permit, with no separate land use permit.
• No minimum lot size for attached or detached ADUs, though the parcel must be a confirmed lot of record.
• No owner-occupancy requirement.
• Conversions. Existing garages, barns, and other accessory buildings can become ADUs even if they don’t meet current setback or lot-coverage standards.
• Alley lots. A detached ADU can sit on a lot line that abuts a public alley.
• Size. Up to 1,248 square feet of floor area, or 1,748 under the county’s density credit program (WCC Chapter 20.91).
• Parking. On lots under 6,000 square feet, no more than one off-street space per unit.
• Design. Per the county, no design or siting standards stricter than those for a single-family home.
• Condominium sale. The county won’t prevent a structure built as an ADU from being sold as a condominium unit separately from the house.
• Services. The parcel must be served by sewer and/or septic, and water must be provided for the ADU.

Outside Urban Growth Areas

The rural rules also apply to UGA lots in the excluded districts.

One ADU per lot, in addition to the existing or permitted house.

Owner occupancy. The owner must occupy one of the dwellings as their primary domicile.

Minimum lot size:

*Or a parcel large enough for two dwelling units under the zone’s gross density.

Plats. Outside UGAs, ADUs are allowed in subdivisions that received preliminary plat approval before January 25, 1994. In subdivisions approved after that date, ADUs are prohibited unless the final plat designates the lot as allowing one. ADUs are never allowed in the reserve tract of a cluster subdivision.

Design and construction:

• The ADU must be clearly subordinate to the house.
• Up to 1,248 square feet of floor area, or 1,748 with density credits.
• Only one access point off a public road may serve both the house and the ADU.
• Locate the ADU to minimize visual impacts; placement close to the house is preferred. A house with an attached ADU may show only one front entrance to the front yard and street; a detached ADU may add only one more entrance visible from the front yard. Fencing or landscaping screens may be required.
• In the Agriculture district, a detached ADU must sit within the farmstead cluster.

Deed restriction. Before the building permit issues, the owner records a deed restriction with the Whatcom County Auditor stating that a detached ADU and its land can’t be financed or sold separately from the house (unless the zone allows that land division), and that the owner occupies one of the units.

Permitting path

The county’s ADU page shows different paths inside and outside UGAs. On eligible UGA lots, the ADU is a permitted use and goes to building permit. Outside UGAs, the county’s chart includes a land use step (an Administrative Use Permit where required) ahead of the building permit. Either way, the county recommends a Land Use Checklist request through its online permit portal, with a site plan and fee, so staff can confirm your pathway and flag submittal items for your property.

Critical areas, water, and septic

The county’s page lists four things to check early: the zone, plat restrictions, regulated critical areas such as wetlands, streams, and habitat, and lot size. Critical areas can shrink or remove the buildable area, and the county notes you may need a qualified natural-resources consultant. State law’s urban protections don’t apply to lots designated with critical areas or their buffers (RCW 36.70A.681).

Water and wastewater come next. The ADU needs water service and a sewer or septic path. On septic, the existing system’s capacity and reserve area often limit the bedroom count and location. Confirm those with the health reviewer before you choose a plan.

How a factory-built ADU fits

Wolf homes are modular, built to Washington’s residential code — not HUD (the federal standard for manufactured homes) manufactured homes. Washington L&I reviews the factory plans and oversees factory inspections, and each home carries its L&I insignia before shipping. The county still reviews zoning, the foundation, utility connections, and site inspections.

The 1,248-square-foot cap covers a wide range of floor plans; compare our models with it. Whatcom County sites are far beyond the standard 20-mile delivery in Wolf’s published installation scope, so transport and setting are quoted after the site review. See cost and the Washington ADU cost guide.

What to have ready

1. Address or parcel number, and whether it is in a city, an eligible UGA lot, or outside a UGA.
2. The zone and lot size, and the recorded plat if the lot came from a subdivision.
3. Any critical-area reports, septic records, and water-service information.
4. How you plan to use the unit, especially outside a UGA where owner occupancy applies.

Send us the address and we’ll confirm which Whatcom County path applies before you pay for design.

This is general information, not legal advice. Recheck the current statute, local code, and permit packet before you file — jurisdictions amend forms and effective dates.