The most common question we hear is also the most important one to answer before you spend a dime: can I even build an ADU on my lot? In Bellingham, the answer changed in a big way in 2025, and most homeowners are still working off old information. This guide explains the current rules under Bellingham Municipal Code 20.10.036 in plain English, so you know what your lot likely allows before you pay for design.
What changed in 2025.
Bellingham updated its ADU ordinance to make units much easier to build. The headline changes:
- Up to two ADUs per single-family lot. Most lots that previously allowed one accessory unit can now have two, for example a garage conversion plus a backyard cottage.
- Detached units up to 1,000 square feet. A detached ADU can now have up to 1,000 square feet of habitable space, enough for a real one or two-bedroom cottage.
- Up to 24 feet tall. The height limit allows a unit with proper ceiling heights and, in some cases, a loft.
- Off-street parking minimums waived. The current ordinance window removes the requirement to add off-street parking for an ADU citywide, which used to kill projects on tight lots.
- Owner-occupancy sunsets July 1, 2026. For most city lots you will no longer have to live on the property to rent the unit, which means you can rent both the house and the ADU.
Together these changes opened ADUs up to a lot of Bellingham homeowners who could not make the old rules work.
What still limits your lot.
The rules are friendlier, but a few things still decide what fits:
- Setbacks. How close a unit can sit to your property lines. On a narrow lot this often decides the cottage's footprint.
- Lot coverage. The total share of your lot that can be covered by buildings. A big main house leaves less room.
- Critical areas. Steep slopes, wetlands, and the Lake Whatcom watershed add rules around stormwater and where you can build.
- Jurisdiction. Inside the city the rules above apply; just outside, in unincorporated Whatcom County or in Lynden and Ferndale, the rules and the path differ.
This is exactly why we start every project with a free feasibility check. We map your lot against the rules and tell you what your lot actually allows before you pay for a single drawing. The design and permitting page explains how that works.
City of Bellingham vs. Whatcom County.
If your property is inside Bellingham city limits, BMC 20.10.036 governs your ADU. If you are in unincorporated Whatcom County, or in a smaller city like Lynden or Ferndale, a different code applies, and the limits on size, height, and number of units can be different. The county also handles its own septic and well rules for rural lots. We know which path your address is on and what each one requires.
What an ADU lets you do.
With the rules loosened, Bellingham homeowners are using ADUs for rental income, for housing parents or adult kids, and for the resale value a permitted unit adds. The owner-occupancy sunset in particular changes the rental math, since you will be able to rent both units. Our owner-occupancy guide covers that change, and our ADU cost guide covers what each type runs.
Frequently asked questions.
Can I build two ADUs on one lot in Bellingham?
For most single-family lots, yes. Under Bellingham's 2025 ordinance update (BMC 20.10.036), many lots can now have up to two ADUs, such as a garage conversion plus a detached backyard cottage. Setbacks and lot coverage decide whether two units physically fit on your specific lot, which our free feasibility check confirms.
How big can an ADU be in Bellingham?
A detached ADU in Bellingham can have up to 1,000 square feet of habitable space and stand up to 24 feet tall under the current rules. The footprint your lot allows still depends on setbacks and lot coverage. Attached and conversion units are governed by the existing structure and the same setback rules.
Do I need to add parking for an ADU in Bellingham?
Not under the current ordinance. Bellingham waived off-street parking minimums for ADUs citywide in the present window, which removed a requirement that used to make units impossible on tight lots. This is one of the bigger changes from the old rules.
How do I find out if my specific lot qualifies?
The fastest way is a feasibility check, which we do for free. We look at your lot size, setbacks, lot coverage, any critical areas, and whether you are in the city or the county, then tell you what your lot allows before you spend anything on design. It is the right first step for any ADU project.
For a free feasibility check and a written, line-item quote, Bellingham ADU Co. handles ADU design and permitting across Bellingham and Whatcom County.