An attached ADU is a separate living unit that shares a wall with your main house but lives on its own. It has its own entrance, its own kitchen, and its own bath, so a parent or an adult kid has real independence and you still have everyone close. We either build a small addition onto the house or carve a suite out of space you already have, like an oversized primary wing or a bonus room over the garage.
Because every house is different, an attached ADU is priced by consultation. The cost depends on whether we add square footage or convert existing space, how far the new kitchen and bath are from existing plumbing, and whether we need to add a separate entrance and sound separation. We handle the zoning, the plans, and the permit with the City of Bellingham or Whatcom County, and we build the suite to current energy code so it is comfortable and quiet for a second household.
What it actually costs.
Attached ADU & in-law suite By consultation
Add vs. convertA new addition costs more than converting existing space
Plumbing distanceA kitchen and bath far from existing lines adds cost
Separate entranceA private door and any sound separation work
Finish levelStandard vs. accessible, aging-in-place finishes
Our four-step process.
Free feasibility check
We look at your house and lot, figure out whether to add on or convert, and give you a realistic budget range.
Design and permit
We draw the suite, plan the entrance and plumbing, and pull the permit with the City or County.
Build the suite
We frame the addition or convert the space, run the kitchen and bath, add the entrance, and finish to code.
Final inspection and walkthrough
We pass final inspection, walk it with you, and close out the punch list.
Why hire a real crew for this.
Its own private entrance, kitchen, and bath
Built by adding on or converting existing space, whichever fits
Sound separation so two households live in peace
Accessible, aging-in-place finishes available
Zoning, plans, and permits handled by us
Frequently asked questions.
How much does an attached ADU or in-law suite cost in Bellingham?
Attached ADUs are priced by consultation because the range is wide and the work depends entirely on your house. Converting an existing bonus room into a suite costs far less than building a new addition with a fresh kitchen and bath. The main drivers are whether we add square footage or convert, how close the new plumbing is to existing lines, and whether we add a separate entrance. We give you a line-item number after a free walkthrough.
What is the difference between an attached ADU and a basement conversion?
An attached ADU is a suite that shares a wall with the main house at grade or as an addition, while a basement conversion turns below-grade space into a unit. They overlap, but a basement conversion has its own challenges like egress windows, ceiling height, and moisture control. We help you weigh both against your house and your budget during the feasibility check.
Can I build an attached ADU for my parents in Bellingham?
Yes, and it is one of the most common reasons homeowners call us. An attached in-law suite with its own entrance, kitchen, and bath lets a parent live independently while staying close. We can add accessible features like a curbless shower, wider doorways, and a no-step entrance. We confirm what your lot and the City or County rules allow before any design work.
Ready for a free attached adu quote?
Tell us about your project and your lot. We'll reply within 15 minutes during business hours.