Basement, Addition or ADU? The Cheapest Way to Add Living Space in Seattle (2026)

The short answer: cheapest to most expensive

• Basement finishing: roughly $60 to $130 per square foot for most projects, and $130 to $180+ for a basement ADU. The structure already exists, so you are paying for finishes and systems only.
• Garage conversion: about $60,000 to $120,000 for a two-car garage turned into living space, or $120,000 to $250,000 as a complete ADU with kitchen and bath. On a per-square-foot basis it sits close to a basement.
• Home addition: $350 to $550 per square foot in most of King County. A 300 to 400 sq ft family-room addition lands around $120,000 to $200,000; a primary suite with bathroom $180,000 to $300,000+; a second story $300,000 to $600,000+.
• Attached ADU: $150,000 to $300,000. Detached ADU (DADU): $250,000 to $450,000+, because it is a small standalone house with its own foundation, roof and utility connections.

Full breakdowns are in our basement cost guide, garage conversion cost guide, home addition cost guide and ADU cost guide.

Why existing space is three to five times cheaper

Foundation, framing, roof and exterior envelope are the expensive parts of any building, and a basement or garage already has them. That is why finishing 900 square feet of basement at $90 to $130 per square foot comes in around $80,000 to $115,000, while adding the same 900 square feet as new construction would run $315,000 to $500,000. The trade-off is that you are working with what the house gives you: ceiling height, window placement, moisture conditions and the slope of a garage slab all have to be solved rather than designed away.

When the basement is the right answer

Choose the basement when you want bedrooms, a family or media room, a home office, a guest suite or a rental unit, and the basement has at least seven feet of ceiling height or can get there. The two cost items that surprise people are egress windows for bedrooms, about $6,000 to $12,000 each installed, and moisture control, which we treat as non-negotiable in Western Washington. A basement with its own entrance is one of the strongest ADU candidates in the region; see our basement finishing page and our basement permit guide for the rules.

When the garage is the right answer

Choose the garage when you can live without covered parking and want a space that is separate from the main living area: a studio, an office, a gym or a one-bedroom ADU. The garage usually has better light and a separate entrance than a basement, and it can be built while the family lives undisturbed upstairs. The cost drivers are leveling the slab, running plumbing, upgrading the electrical panel and insulating to the Washington State Energy Code. Our garage conversion page and our garage conversion vs. DADU comparison cover the details.

When an addition is the right answer

Choose an addition when the problem is the main house itself: a kitchen that is too small, no primary suite, a rambler that needs a second story, or a family room that opens to the yard. An addition keeps everything under one roof and adds finished square footage that appraisers compare directly against neighborhood comps. It costs more per square foot than any conversion, but it is the only option that changes how the main house lives day to day. See our home additions page and, if you are weighing it against a second unit, our addition vs. ADU guide.

When an ADU is the right answer

Choose an ADU when the space needs independence: an aging parent, an adult child, a long-term renter, or a place to downsize into later. Under Washington's HB 1337, most lots in Seattle, Bellevue, Kirkland, Redmond and other King County cities can now host at least two ADUs, with relaxed parking and owner-occupancy rules. A detached unit near Seattle or Bellevue job centers commonly rents for $1,800 to $3,200+ per month, which is why many clients treat a DADU as an investment rather than an expense. Start on our ADU page or the DADU page.

Timelines and permits compared

Basement finishing and garage conversions typically need a few weeks to a couple of months of permitting and 2 to 4 months of construction. Additions need engineering and a full building permit, with 2 to 4 months of design and permitting followed by 4 to 8 months of construction, longer in Seattle than in most Eastside cities. A new DADU adds siting, setbacks and utility connections and usually runs 8 to 14 months start to finish. Our Seattle vs. Eastside permit timeline guide shows what to expect city by city, and every option needs a building permit; none of them is a cosmetic project.

A simple way to decide

1. Name the problem. More bedrooms, a better main floor, or an independent unit? Each points to a different option.
2. Check what you already own. Ceiling height and moisture in the basement; slab, structure and location of the garage; lot coverage and setbacks for an addition or DADU.
3. Price the space you will actually use. A basement that becomes a storage room is not cheaper than an addition you live in every day.
4. Get fixed bids on two options. The comparison is only real when both numbers are complete; our guide to comparing contractor bids shows how.

Price two options with one builder

Blue Lake Construction designs and builds basement finishes, garage conversions, additions, ADUs and DADUs across Seattle, Bellevue, Kirkland, Redmond, Sammamish, Issaquah, Renton, Mercer Island and Kent, all on fixed-price, fixed-duration contracts under Washington contractor license BLUELLC760C3. Request a free consultation and we will walk your house and lot, flag the issues that decide the choice, and price the two best options as fixed bids so you can compare them honestly.