Learn more about ADUs and how to build them in Bellingham
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What is an ADU?
An ADU is a separate, self-contained residential unit located on the same lot as a principal home on a property – that principal home can be a single family, duplex, triplex, fourplex, townhome, or other principal unit. While ADUs are generally smaller in size and prominence, they have all the basic facilities needed for day-to-day living independent of the principal home, including a kitchen, sleeping area and a bathroom.
Supporting our community’s housing needs
Bellingham has formally allowed ADUs since 1995, when the City passed its first ADU ordinance. ADUs are seen as a valuable component in achieving the City’s infill housing goals. There are many benefits to enabling ADU development in Bellingham including increasing housing choice and affordability as well as providing an opportunity for homeowners to earn additional income, age in place, or care for an ailing relative.
Types of ADUs
Generally, we divide ADUs into two categories:
- Attached ADUs (A-ADUs): As their name suggests, A-ADUs are attached to the principal residence on a property.
- Detached ADUs (D-ADUs): D-ADUs are detached from the principal residence.
ADUs can be configured on properties in many ways, including:
- A separate structure on the lot (such as a converted garage).
- A separate unit within an existing home or principal unit (such as in an attic or basement).
- An addition to a home or principal unit (such as a separate apartment unit with a separate entrance).
Example Configurations
The diagrams shown above are just some examples of possible ADU configurations. Many configurations are possible. Different building codes may be applicable (IBC vs IRC) depending on the design and configuration. Please consult with the Permit Center to discuss the feasibility of your project.
Considering putting an ADU on your property?
If you are considering building an ADU in Bellingham, you can find an overview of our ADU regulations in the table below. Additional details can be found in Bellingham Municipal Code 20.10.036.
ADU Development Standards
| Attached ADU | Detached ADU | |
|---|---|---|
| Where in Bellingham are ADUs allowed? | All areas that allow residential development (except for the Lake Whatcom watershed), including areas zoned residential single and residential multi, and in urban villages | Same as Attached ADU |
| Permits required | Land Use and Building; must have an approved land use permit prior to building permit issuance | Same |
| Site layout | Allowed on a lot with a single-family dwelling unit, duplex, triplex, fourplex, townhome, or other principal unit | Same |
| Number per lot – max | 2 | Same |
| Size – max | 1,000 sq/ft | 1,000 sq/ft * |
| Height – max | 35′ under height definition #1, or 20′ under height definition #2 (see definitions) | 24′ under height definition #1, or 12′ under height definition #2 (see definitions) |
| Setback from property line – min (see diagram) | Front: 10’ Rear: 10’ (if alley, then 0′) Corner lot side flanking: 10’ Interior lot side: 5’ (if alley, then 0′) | Front: 10’ Rear: 0’ Corner lot side flanking: 10’ Interior lot side: 5’ (if alley, then 0′), and a DADU may be located in the rear 32’ without a side setback in that section |
| Parking – min** | None required | Same |
| Open space – min | 30% of lot | Same |
| Permitting process | A Type I permit process is used for the majority of ADUs (see when other processes would be used instead) | Same |
| Owner occupancy | Not required | Same |
| Sales | May be sold as a condominium unit independent of the principal unit | Same |
| Subdivision of land | Not allowed | Allowed under specific circumstances*** |
**Parking added electively must meet development standards per BMC 20.10.036(B)(9) and 20.12.010.
***May be subdivided onto lots separate form the principal unit if the lots the DADUs are on are deed restricted to provide affordable owner-occupied units.
What is the permitting process for ADUs?
If you would like to put an ADU on your property, you must submit both land use and building permit applications (the land use permit must be approved prior to building permit issuance). While the process for reviewing permit applications can vary, the vast majority of ADUs in Bellingham go through a Type I permit review process.
Which permit process will my project require?
|
Type I Process |
Type II Process |
Type III-A/Conditional Use |
|---|---|---|
|
Used for the vast majority of ADUs in Bellingham that meet standards in the ADU Development Standards table.* See Type II and III to the right to learn when they would be used instead. |
Required when a project consists of 7 or more dwelling units, including ADUs. |
If a D-ADU is proposed to exceed the maximum size limits, a Conditional Use Permit is required under this process. A Type III process is more complex and adds review time. To avoid this consider redesigning the ADU to an Infill Housing type in order stay within a Type I or Type II process. |
|
This is the most streamlined permit review process. |
This process is slightly more time consuming for applicants because notification of surrounding property owners is required. |
This ADU permit process is the most time consuming for applicants because it requires notification of surrounding property owners, a neighborhood meeting, and a public hearing. |
*Even if your project includes request for minor modifications from applicable development standards, you will likely qualify for the Type I Process.
Frequently Asked Questions
There is one set of registered, pre-approved ADU plans available in Bellingham. What this means for ADU applicants:
- Both a land use and building permit application are still required.
- The building permit review process should be faster and less expensive than it would be without the use of the registered plans.
If you are considering using a pre-approved plan, please contact the Permit Center regarding site planning for the ADU. Registered plans can be purchased from the architect of record. Design details can be found here.
Homeowners with existing non-permitted ADUs must apply to make them legal. Units that are not legalized will be subject to enforcement. Pre-existing units must meet the same requirements as new ADUs, including building code standards.
Note: Allowing more than one dwelling unit on property in single-family zones is not limited to ADUs. Ordinance 2025-12-032 applies interim development regulations to single-family and infill housing development, allowing infill housing (up to 6 units per lot) in all areas that are zoned predominantly for residential use, except on property in the Lake Whatcom Reservoir. The ordinance went into effect 12/23/2025 and will be in effect until December 4, 2026, absent further action by the City Council. Applications for ADUs and Infill Housing are available at the Permit Center.
Yes, all rented ADUs within City limits must register with the Rental Registration program. While they must all be registered, fee and inspection requirements vary:
- Attached ADUs are exempt from fees and inspection requirements if:
- There are no other rental units on the property, and
- The owner resides in the principal unit on the property or in the ADU.
- Detached ADUs are not exempt from fees and inspection requirements.
Learn more about the Rental Registration and Safety Inspection Program and register your ADU.
Watch to learn more about ADUs in Bellingham
This panel discussion provides a comprehensive overview of ADUs in Bellingham, covering key aspects of financing, permitting, construction, rental, and sale.
Additional resources
- AARP: The ABC’s of ADU’s
- AccessoryDwellings.org – Portland (OR)-based website maintained by volunteers provides current ADU news, articles, project examples, and other resources
- Accessory Dwelling Units 2022 Annual Report (3/2023 – City of Seattle)
- Aging in Place: An Intergenerational Priority
- Municipal Research and Services Center (MRSC) information page on ADUs
