The rules in one table
| Rule | What Bellingham says |
|---|---|
| Where | All areas that allow residential development, including urban villages, except the Lake Whatcom watershed |
| How many | Up to 2 per lot |
| Size | 1,000 sq ft, attached or detached; a detached ADU may add up to 800 sq ft of attached ancillary space (garage, shed, workshop) |
| Height | Attached: 35 ft or 20 ft; detached: 24 ft or 12 ft, depending on which of the City's two height definitions applies |
| Owner occupancy | Not required; older covenants can be released |
| Parking | None required; any you add must meet BMC 20.10.036(B)(9) and 20.12.010 |
| Renting it | "All rented ADUs within City limits must register with the Rental Registration program" |
| Permits | Land use permit, then building permit |
| Pre-approved plans | One registered plan set, bought from its architect |
Source: the City's ADU page, which summarises Interim Ordinance 2025-12-032. The interim rules lapse on 4 December 2026 unless Council acts, so check the page again before you pay for plans; the full code is BMC 20.10.036.
Which review your ADU gets
| Review | When | What it means |
|---|---|---|
| Type I | Most ADUs | Staff decision, the fastest |
| Type II | The project has 7 or more dwelling units, ADUs included | Neighbour notice, slower |
| Type III-A | A detached ADU over the size limits in BMC 20.10.036(B)(6) | Conditional use permit with a public hearing, the slowest |
With the City's registered plan set, "the building permit review process should be faster and less expensive", but both permits are still required.
What it means for hiring a builder
- Ask who handles the land use permit. It comes before the building permit, and some builders price only construction. Get a designer or builder who has taken an ADU through both.
- Licensing. Washington requires contractors to be registered with L&I; check the registration and bond on the day you sign (see checking a contractor license in Washington).
- Sewer. If the ADU needs a new or extended side sewer, the owner-responsibility and licensed-contractor rules are in Bellingham side sewer repair.
- If you will rent it, budget for rental registration and its inspection rules from the start.
What 110 Bellingham ADU permits show
| Measure | Bellingham ADU |
|---|---|
| Permits on homes, October 2023 to September 2026 | 110 |
| Firms / active in the last 12 months | 71 / 33 |
| Usual filed value | $140,755 |
| Middle half of builders' usual value (11 builders with 3+ permits) | $118,076 to $152,242 |
| Usual days from issue to final | 329 |
| Permits that reached a final | 79.2% |
| Firms with a single ADU permit | 51 (71.8%) |
| Firms with an active state license match | 83.1% |
| Busiest / quietest month to file | March (174) / May (33) |
The usual ADU took 329 days from issue to final, close to a new house (326 days) and an addition (328). Plan on most of a year of construction once the building permit is out. No builder dominates: the five busiest hold 20.9% of ADU permits, and 71.8% of firms filed just one. Ask for a finished ADU's address and look up its permit in eTRAKiT (Bellingham building permit search).
Values by firm are on Bellingham ADU cost and, for comparison, Bellingham addition cost.
Compare Bellingham ADU builders by permits →
What this does not tell you
Filed values are construction estimates on the application and leave out design, land use fees and utility connection charges; the City ADU page does not address impact fees. Our permits mostly predate the December 2025 interim ordinance, so they reflect the older rules. We count building permits, not the land use permits that come first.
How we know this
Rules are quoted from the City of Bellingham's ADU page, which summarises Interim Ordinance 2025-12-032, read 23 October 2026. Permit counts are City of Bellingham permits on homes classified as ADUs, October 2023 to September 2026; license matches are to Washington L&I contractor records.
Numbers come from City of Bellingham permits on single-family homes, duplexes and townhomes, data through October 15, 2026. Nobody paid to be on this page and nobody can.
Questions
- How many ADUs can I have in Bellingham?
Up to two per lot, attached or detached, in all areas that allow residential development except the Lake Whatcom watershed, under Interim Ordinance 2025-12-032 (in effect from 23 December 2025 until 4 December 2026 unless Council acts).
- How big can an ADU be in Bellingham?
1,000 square feet for attached and detached ADUs. A detached ADU may also have up to 800 square feet of attached ancillary space such as a garage, shed or workshop. Going over the size limits needs a Type III-A conditional use permit with a public hearing.
- Do I have to live on the property to have an ADU in Bellingham?
No. The City says owner occupancy is not required, and owners with an older recorded owner-occupancy covenant can ask to release it with the City's release form.
- Do I need a permit to build an ADU in Bellingham?
Two: a land use permit and a building permit, and the land use permit must be approved before the building permit is issued. Most ADUs are Type I, decided by staff.
- How long does it take to build an ADU in Bellingham?
The usual Bellingham ADU permit in our data took 329 days from issue to final, and 79.2% of ADU permits reached a final. That clock starts after the land use permit, so add design and review time before it.
Build your own list
Every firm with that kind of permit on record. Pick what you value. The list moves.
Related guides
- Can an unpermitted ADU be legalized? California's AB 2533 rule, and what it costs and takes
- Denver ADU rules since citywide ADUs: owner occupancy, the licensed-contractor rule, and what ADU permits were filed at
- Do I need a permit for an ADU? Yes; here is what 9 cities require, and how long it takes
- Nashville DADU rules: where a backyard cottage is allowed, the 700/850 sq ft limits, and who can pull the permit
- Do I need an HVAC permit in Dallas? The mechanical permit, who can pull it, and 8,582 permits
- Do I need a plumbing permit in Dallas? Licensed plumber or homestead owner, and 19,009 permits
For one city
- Accessory Dwelling Units (ADUs) — City of Bellingham Planning (accessed 2026-10-23)
- BMC 20.10.036 Accessory dwelling units — Bellingham Municipal Code (accessed 2026-10-23)