ADU requirements differ by jurisdiction. The City of Bellingham allows up to 2 ADUs per lot, each up to 1,000 sf, with no owner-occupancy or parking requirement. Rural unincorporated Whatcom County allows 1 ADU of up to 1,248 sf and requires owner-occupancy. Whatcom County UGAs allow up to 2 ADUs without owner-occupancy.
Sources: BMC 20.10.036, as amended by Ordinance 2025-12-035 (effective 2025-12-23); WCC 20.80.910. All rows last verified 2026-10-05. This page is the reference that other pages on this site link to for ADU numbers.
How to Read This Table
- Each row is one rule. Each column is one jurisdiction.
- The City of Bellingham applies inside city limits. Unincorporated Whatcom County applies outside city limits, and splits into rural parcels and Urban Growth Area (UGA) parcels.
- Each cell gives the standard, then the code section it comes from.
- “Not in our verified extract” means the standard exists in code but was not confirmed for this table. It does not mean there is no rule.
- Small cities in Whatcom County (Ferndale, Lynden, Blaine, Everson, Nooksack, Sumas) have their own codes and are not covered here.
ADU Requirements by Jurisdiction
| Rule | City of Bellingham | Unincorporated Whatcom — rural (outside a UGA) | Unincorporated Whatcom — UGA | Last verified |
|---|---|---|---|---|
| ADUs allowed per lot | Up to 2, attached or detached. ADUs are exempt from density calculations. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | 1. WCC 20.80.910 | Up to 2. WCC 20.80.910 | |
| Owner-occupancy | Not required. Repealed effective 2025-12-23. A previously recorded covenant needs an owner-filed release. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | Required. WCC 20.80.910 | Not required. WCC 20.80.910 | |
| Maximum ADU size | 1,000 sf per ADU. A basement attached ADU is exempt from this cap where the owner has occupied the primary residence 5+ years. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | 1,248 sf; 1,748 sf with a density credit. WCC 20.80.910 | 1,248 sf; 1,748 sf with a density credit (county standard). WCC 20.80.910 | |
| Unconditioned ancillary space | Up to 800 sf may attach to one detached ADU. A conditional use permit is required above 1,800 sf combined (2,000 sf for two ADUs). BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | |
| Maximum height | Detached: 24 ft (12 ft condition). Attached: 35 ft (20 ft condition). BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | 35 ft in rural districts. WCC 20.80.910 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | |
| Setbacks, detached ADU | Front 10 ft. Rear 0 ft. Side 5 ft, or 0 ft abutting an alley. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.210 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.210 | |
| Rear free-placement strip | A detached ADU may be placed within 32 ft of the rear lot line. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | No equivalent provision in our verified extract. WCC 20.80.910 | No equivalent provision in our verified extract. WCC 20.80.910 | |
| Open space and lot coverage | 30% of the lot must be open space. No ADU-specific citywide lot-coverage or impervious cap. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | |
| Off-street parking | None required. Citywide parking reform made permanent August 2026. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | Set by WCC 20.80.580(63). Amount not in our verified extract. WCC 20.80.580(63) | Set by WCC 20.80.580(63). Amount not in our verified extract. WCC 20.80.580(63) | |
| Minimum parcel size, detached ADU | No acreage minimum is listed among the BMC 20.10.036 ADU standards. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | 4.5 ac rural; 10 ac RR-I; 10 ac Lake Whatcom Watershed Overlay; 20 ac Rural Forestry; 40 ac Agricultural. WCC 20.80.910 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | |
| Permit pathway | ADU land-use review (Type I or Type II), then a building permit. City Building Permit Fee Worksheet BSD#064 (2026-08-27) | Administrative Approval Use (ADM), a Type II process. County-published processing time up to 100 days. Approval valid 2 years. Then a building permit. WCC 22.05.028 | Permitted use. Apply directly for a building permit; no land-use permit. WCC 20.80.910 | |
| Districts where the rule set applies | Inside City of Bellingham limits, except the Lake Whatcom Reservoir overlay (see below). BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | Rural parcels outside an Urban Growth Area. WCC 20.80.910 | UGA parcels, excluding the LWWO, GC, AG, R and RF districts. WCC 20.80.910 | |
| Lake Whatcom watershed | New ADUs are barred in the Lake Whatcom Reservoir overlay, with legacy pre-1995/2001 exceptions. Impervious limits of 2,000–2,500 sf or 25%. BMC 16.80 | Lake Whatcom Watershed Overlay: 10 ac minimum for a detached ADU. WCC 20.80.910 | LWWO parcels are excluded from the two-ADU UGA rule. WCC 20.80.910 | |
| Detached garage conversion | An existing detached garage may convert to a detached ADU even where it does not meet current setback or lot-coverage standards. No replacement parking. Current building, egress, fire-separation and energy code apply. BMC 20.10.036(B)(15), as amended by Ord. 2025-12-035 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | |
| Sale of ADU as a condominium unit | Allowed. BMC 20.10.036, as amended by Ord. 2025-12-035 (eff. 2025-12-23) | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 20.80.910 | |
| Historic districts | Eldridge, South Hill and Sehome require a Certificate of Alteration (Type VII). BMC Ch. 17.90 | Not applicable (City provision). BMC Ch. 17.90 | Not applicable (City provision). BMC Ch. 17.90 | |
| On-site sewage (septic) capacity | Not applicable to lots on City sewer. WCC Ch. 24.05 (on-site sewage systems only) | Minimum design flow for an additional dwelling is 120 gal per bedroom. A system at its permitted bedroom count needs a new system for the ADU; a bedroom may not be reassigned from the main house. 100% reserve area must stay clear of the ADU, driveway and parking. WCC Ch. 24.05; Ch. 246-272A WAC (amended eff. 2025-04-01) | Same rules apply where the parcel is on septic. WCC Ch. 24.05; Ch. 246-272A WAC | |
| Fire access (rural driveways) | The 2021 IFC applies statewide. This row covers only the county rural driveway standards. 2021 IFC via Ch. 51-54A WAC | Driveway 12 ft minimum width serving up to two residential lots. Dead-end access over 150 ft requires an approved turnaround. Grades of 12% or steeper require mitigation. 2021 IFC via Ch. 51-54A WAC, adopted at WCC 15.04.010(C) | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. WCC 15.04.010(C) | |
| Impact fees | Park impact fee $1,470.93 per ADU (exemptions possible under BMC 19.04). Transportation impact fee reduced by a 50% ADU credit, not waived. School impact fee N/A. City Building Permit Fee Worksheet BSD#064 (2026-08-27) | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. 2025–2026 Unified Fee Schedule, Ord. 2024-063 | Not in our verified extract. Confirm with Whatcom County Planning & Development Services. 2025–2026 Unified Fee Schedule, Ord. 2024-063 | |
| Governing fee schedule | Land use ADU Type I $294, Type II $728 (effective 2026-10-01). Building permit from valuation: $177.63/sf new living space. Plan review 65% of permit fee. City Building Permit Fee Worksheet BSD#064 (2026-08-27) | 2025–2026 Unified Fee Schedule, in force through 2026-12-31. Ord. 2024-063 | 2025–2026 Unified Fee Schedule, in force through 2026-12-31. Ord. 2024-063 | |
| Energy code | 2021 WSEC: heat-pump space and water heating required in new residential. Initiative 2066 struck down 2026-09-17. 2021 WSEC (eff. 2024-03-15) | 2021 WSEC: heat-pump space and water heating required in new residential. Initiative 2066 struck down 2026-09-17. 2021 WSEC (eff. 2024-03-15) | 2021 WSEC: heat-pump space and water heating required in new residential. Initiative 2066 struck down 2026-09-17. 2021 WSEC (eff. 2024-03-15) |
Table last verified 2026-10-05. Not legal advice. Codes change; confirm with City of Bellingham Planning or Whatcom County Planning & Development Services before you design.
Which Column Is Your Parcel In?
The answer depends on city limits, the UGA boundary, the zoning district and any overlay. The feasibility study checks those for a specific parcel.
Check My ParcelThe Differences That Decide Most Projects
Owner-occupancy. The City of Bellingham repealed its owner-occupancy requirement effective 2025-12-23 (BMC 20.10.036, as amended by Ordinance 2025-12-035). Whatcom County UGA parcels have no owner-occupancy requirement. Rural parcels outside a UGA still require the owner to live on the property (WCC 20.80.910). On a rural parcel, a plan to rent both the main house and the ADU does not fit this rule.
Number of ADUs. The City of Bellingham and Whatcom County UGAs allow up to 2 ADUs per lot. Rural parcels allow 1 (BMC 20.10.036; WCC 20.80.910).
Size. The City of Bellingham caps each ADU at 1,000 sf. Unincorporated Whatcom County caps an ADU at 1,248 sf, or 1,748 sf with a density credit (BMC 20.10.036; WCC 20.80.910).
Detached ADUs on rural land. A detached ADU in unincorporated Whatcom County requires a minimum parcel size: 4.5 acres in rural districts and more in RR-I, Rural Forestry, Agricultural and the Lake Whatcom Watershed Overlay (WCC 20.80.910).
Permit path. A City of Bellingham ADU goes through an ADU land-use review and then a building permit. A rural county ADU goes through an Administrative Approval Use (ADM), a Type II process with a county-published processing time of up to 100 days (WCC 22.05.028). A UGA ADU is a permitted use and goes straight to a building permit (WCC 20.80.910).
Detail Behind Each Column
For the City of Bellingham, zoning-district detail, lot examples and the full set of BMC 20.10.036 standards are in the Bellingham ADU zoning guide.
For unincorporated parcels, the rural and UGA provisions of WCC 20.80.910 are explained in the Whatcom County ADU regulations guide.
For a narrative comparison of the two jurisdictions, including which one governs a parcel near the city edge, see Bellingham vs Whatcom County ADU rules compared.
Frequently Asked Questions
What are the ADU rules in Bellingham?
In the City of Bellingham, a lot may have up to 2 ADUs, attached or detached, each up to 1,000 sf. Detached ADUs may be 24 ft tall and attached ADUs 35 ft. Detached setbacks are 10 ft front, 0 ft rear and 5 ft side (0 ft at an alley). No parking is required, owner-occupancy was repealed effective 2025-12-23, and an ADU may be sold as a condominium unit. Source: BMC 20.10.036, as amended by Ordinance 2025-12-035, effective 2025-12-23. Last verified 2026-10-05.
What are the ADU rules in unincorporated Whatcom County?
It depends on whether the parcel is inside an Urban Growth Area. A rural parcel outside a UGA may have 1 ADU, the owner must live on the property, and approval runs through an Administrative Approval Use (ADM) permit. A UGA parcel, excluding the LWWO, GC, AG, R and RF districts, may have up to 2 ADUs with no owner-occupancy requirement, and the ADU is a permitted use. The county maximum size is 1,248 sf, or 1,748 sf with a density credit. Source: WCC 20.80.910; WCC 22.05.028. Last verified 2026-10-05.
How big can an ADU be in Whatcom County?
In the City of Bellingham, the maximum is 1,000 sf per ADU (BMC 20.10.036, as amended by Ordinance 2025-12-035). In unincorporated Whatcom County, the maximum is 1,248 sf, rising to 1,748 sf with a density credit (WCC 20.80.910). Last verified 2026-10-05.
Do you need acreage to build a detached ADU in rural Whatcom County?
Yes. In unincorporated Whatcom County, a detached ADU requires a minimum of 4.5 acres in rural districts, 10 acres in RR-I, 10 acres in the Lake Whatcom Watershed Overlay, 20 acres in Rural Forestry and 40 acres in Agricultural districts. Source: WCC 20.80.910. Last verified 2026-10-05.
Can you build an ADU in the Lake Whatcom watershed?
Usually not. In the City of Bellingham, BMC 16.80 bars new ADUs in the Lake Whatcom Reservoir overlay, with narrow legacy pre-1995/2001 exceptions. In unincorporated Whatcom County, a detached ADU in the Lake Whatcom Watershed Overlay requires 10 acres (WCC 20.80.910). Last verified 2026-10-05.
Last verified: October 5, 2026. Not legal advice. Verify current regulations with City of Bellingham Planning or Whatcom County Planning & Development Services.
Find Out Which Rules Apply to Your Parcel
The feasibility study identifies the jurisdiction, zoning district and overlays for one parcel and applies the matching column of this table.
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