A detached ADU, sometimes called a DADU or a backyard cottage, is the most expensive type of accessory dwelling unit to build, and also the one most people picture when they imagine an ADU. In Whatcom County in 2026, a detached ADU costs $250,000 to $400,000 or more, which works out to roughly $400 to $550 per square foot. This guide explains why a backyard cottage costs what it does, the line items behind the number, and how to budget for one.
Why a detached cottage is priced like a small house.
The reason a detached ADU costs so much more than a garage or basement conversion is that nothing is there yet. You are building a complete small house from scratch: a foundation, framing, a roof, siding, windows, a full kitchen and bathroom, heating, and brand-new water, sewer, and power runs trenched out from the main house. A conversion reuses an existing shell. A detached cottage builds everything.
That is also why the cost per square foot is higher than a typical home build. A 2,500-square-foot house spreads its kitchen and utility costs across a lot of square footage. A 600-square-foot cottage has the same kitchen and the same utility connections packed into a quarter of the space, so each square foot carries more of the fixed cost.
The line items behind a backyard cottage.
Here is roughly how the budget breaks down on a typical Whatcom County detached ADU:
| Line item | Share of cost | Notes |
|---|---|---|
| Foundation and site work | 10% to 18% | More on sloped or wooded lots |
| Framing, roof, siding, windows | 25% to 35% | The shell of the cottage |
| Kitchen and bathroom | 15% to 22% | Cabinets, fixtures, plumbing |
| Utility runs and connections | 10% to 20% | Sewer, water, power to the unit |
| Electrical, heating, insulation | 12% to 18% | Heat pump, energy-code insulation |
| Design, permits, fees | 6% to 12% | Often rolled into the build |
What drives the range in Whatcom County.
- Size. Under Bellingham's 2025 rules a detached unit can reach 1,000 square feet. A studio near 400 square feet lands far below a full two-bedroom near the cap.
- Site access and slope. A flat, open lot in Ferndale or Barkley is cheaper to build on than a steep, wooded lot in Sudden Valley where access drives the cost.
- Utility distance. The farther the cottage sits from your sewer, water, and panel, the more trenching and connection work it takes.
- Finish level. A durable rental-grade finish costs less than owner-quality cabinets, flooring, and fixtures.
- Jurisdiction. City of Bellingham lots and unincorporated Whatcom County lots have different fees, stormwater rules, and sometimes septic instead of sewer.
Our detached ADU build page covers the process, and the utility and site work page explains the trenching and foundation work that often swings the budget the most.
City vs. county: the jurisdiction factor.
Where your lot sits changes the path and some of the cost. Inside Bellingham city limits, your cottage runs through the City, ties into city sewer and water, and follows city stormwater rules. Out in unincorporated Whatcom County, or in places like Lynden and Ferndale's rural edges, a cottage may run on a septic system and a well, which changes the utility planning entirely. We confirm your jurisdiction during the free feasibility check, because it affects both the budget and the timeline. Our permit timeline guide walks through both paths.
How to budget for a detached ADU.
Plan for the construction cost plus a cushion. Beyond the $250,000 to $400,000 build range, budget for impact and connection fees, any electrical service upgrade, and a contingency of 10 to 15 percent for the surprises that older properties and wet ground tend to deliver. A good builder gives you a line-item quote after a site visit, not a number over the phone, so you can see exactly where every dollar goes.
Frequently asked questions.
How much does a detached ADU cost in Whatcom County?
A ground-up detached ADU in Whatcom County costs $250,000 to $400,000 or more in 2026, or roughly $400 to $550 per square foot. A small studio lands at the lower end and a 1,000-square-foot two-bedroom cottage with quality finishes and long utility runs lands at the top. The unit is a complete small house, which is why it costs more than a conversion.
Why is a detached ADU more expensive per square foot than a house?
Because the fixed costs of a kitchen, a bathroom, and utility connections spread across fewer square feet. A small cottage needs all the same systems a house does, just in a quarter of the space, so each square foot carries more of that cost. Site access and utility distance on a backyard lot add to it.
Can I build a 1,000 square foot backyard cottage in Bellingham?
Yes, under Bellingham's 2025 ordinance update a detached ADU can have up to 1,000 square feet of habitable space and stand up to 24 feet tall, and most single-family lots can now have up to two ADUs. Whether your specific lot supports a unit that size depends on setbacks and lot coverage, which our free feasibility check confirms.
What is the most expensive part of building a backyard cottage?
It is usually a tie between the shell (framing, roof, siding, windows) and the combination of utility runs and site work, especially on a sloped or wooded Whatcom County lot. The kitchen and bath are also a big share. The farther the cottage is from existing utilities and the harder the site is to access, the more those line items grow.
For a free feasibility check and a written, line-item quote, Bellingham ADU Co. handles detached ADUs across Bellingham and Whatcom County.