How Much Does an ADU Cost in Bellingham in 2026?

If you are thinking about adding an accessory dwelling unit in Bellingham, the first question is almost always the same: what does it cost? The honest answer is that it depends heavily on the type of unit, but the ranges are knowable. In 2026, an ADU in Bellingham runs anywhere from $80,000 for a basic garage conversion to $400,000 or more for a ground-up detached cottage. This guide breaks down the real numbers by type, the cost per square foot, and the specific line items that swing your total up or down.

ADU cost in Bellingham by type.

The single biggest factor in your cost is which kind of ADU you build. Here are the 2026 ranges we see across Bellingham and Whatcom County:

A garage conversion is usually the cheapest path because the structure already exists. A ground-up detached cottage is the most expensive because you are building a small house from the foundation up, with its own utilities. A basement or attached conversion lands in between, with its own quirks like egress and moisture work.

Why a detached cottage costs so much more.

A new detached ADU in Bellingham runs $250,000 to $400,000 or more, which surprises people who expected something closer to a remodel. The reason is simple: a detached unit is a complete small house. You are paying for a foundation, framing, a roof, siding, windows, a full kitchen and bathroom, heating, and brand-new water, sewer, and power runs trenched out to the unit. None of that exists yet, unlike a conversion where the walls and roof are already standing.

At $400 to $550 per square foot, a 600-square-foot cottage lands around $240,000 to $330,000, and a 1,000-square-foot two-bedroom can push past $400,000 once you add quality finishes and long utility runs. The detached ADU build page walks through the full scope, and our detached ADU cost guide for Whatcom County breaks the line items down further.

What drives the number up or down.

Within each type, a handful of factors decide where you land in the range:

• Size. The most obvious lever. A studio costs far less than a two-bedroom, both in materials and in the kitchen and bath that anchor the price.
• Finish level. A durable rental finish costs less than owner-quality cabinets, flooring, and fixtures. The gap can be $20,000 or more on a cottage.
• Site access. A tight or sloped Bellingham lot that needs a crane or hand-carried materials adds labor. Fairhaven and Sehome lots see this often.
• Utility runs. The distance from your existing sewer, water, and electrical panel to the unit sets the trenching cost. A detached unit far from the house is the priciest case.
• Moisture and slab work. On a garage or basement conversion, dealing with an old slab and our wet climate is real money but not optional.

The extra costs people forget.

The construction cost is not the whole picture. Budget for these too:

• Design and permitting: $8,000 to $25,000, though this often folds into the build when you hire one team. See our ADU permit guide.
• Impact and connection fees: the City of Bellingham or Whatcom County charges fees for a new unit's sewer and water connection. These vary by address.
• Site and stormwater work: grading, drainage, and the stormwater management your lot requires, especially near the Lake Whatcom watershed.
• Utility upgrades: an older home's electrical panel often needs an upgrade to carry a second kitchen and a heat pump.

Does an ADU pencil out in Bellingham?

Whether the math works depends on your goal. As a rental, an ADU brings in monthly income and adds resale value, and with Bellingham's owner-occupancy rule sunsetting July 1, 2026, most city homeowners will be able to rent both the main house and the unit. As multi-generational housing, the comparison is against the cost of assisted living or a separate household. We help you run honest numbers before you commit, weighing build cost against rent, resale, and the alternative. Our guide to the 2026 owner-occupancy sunset explains the rule change in plain English.

Frequently asked questions.

What is the cheapest type of ADU to build in Bellingham?

An attached garage conversion is usually the cheapest, starting around $80,000, because the structure and utilities are already close. The walls and roof exist, so you are paying mainly for insulation, a kitchen and bath, egress, and the electrical and plumbing to make it a legal unit. A detached garage costs a bit more because utilities have to be run out to it.

How much does a detached ADU cost per square foot in Bellingham?

A ground-up detached ADU in Bellingham runs roughly $400 to $550 per square foot in 2026. That is higher than a typical house build because a small unit still needs a full kitchen, a bathroom, and its own utility connections, and those fixed costs spread across fewer square feet. Finish level and site access push the number within that range.

Are there extra fees beyond the construction cost?

Yes. Plan for design and permitting ($8,000 to $25,000, often rolled into the build), impact and connection fees charged by the City of Bellingham or Whatcom County for a new unit, and any electrical panel upgrade your older home needs. We list all of these in the line-item quote so there are no surprises.

Will an ADU add value to my Bellingham home?

Generally yes. A legal, permitted ADU adds usable square footage and rental potential, both of which buyers and appraisers value, and the 2026 rule changes that allow two ADUs per lot and remove parking minimums make units easier to build and rent. The exact value lift depends on the unit and the local market, which is part of the math we help you run.

For a free feasibility check and a written, line-item quote, Bellingham ADU Co. handles detached ADUs across Bellingham and Whatcom County.