Home Addition Cost Bellevue WA (2026) | Northwest Custom Homes

The short answer

From a $36,000 bump-out to a $270,000+ second story

Typical 2026 ranges for home additions in Bellevue and on the Eastside: a bump-out $36,000–$90,000 (1–2 months on site), a room addition $80,000–$160,000 (3–5 months), a primary suite addition $135,000–$270,000 (4–6 months), and a second story from $270,000 (6–9 months). Design, engineering and permit review come before construction and add time on the front end.

Price ranges

Home addition costs at a glance

Addition type	Typical size	Typical price	Construction
Bump-out	50–150 sq ft	$36,000–$90,000	1–2 months
Room addition	200–400 sq ft	$80,000–$160,000	3–5 months
Primary suite addition	350–600 sq ft	$135,000–$270,000	4–6 months
Second-story addition	600+ sq ft	$270,000+	6–9 months

Ranges include structure, finishes and standard permits. Engineering, utility upgrades and difficult sites are priced after we see the lot. Your price is set by a written estimate after a free walkthrough.

What we build

Types of additions

Room additions

A bedroom, family room, home office or a bigger kitchen.

Primary suite additions

Bedroom, bathroom and walk-in closet as one wing.

Second stories

Double the living space without giving up the yard.

Bump-outs

A few extra feet for a kitchen, bathroom or dining nook.

Garage and basement conversions

Turning space you already have into living space or an ADU — often the cheapest square footage on the lot.

What moves the number

What drives addition cost

Foundation and site. Slab or crawlspace, slope, soil and drainage. Hillside lots on the Eastside cost more to build on.
Tying into the house. Matching the roofline and siding, and opening exterior walls.
What goes inside. A bedroom costs far less per square foot than a bathroom or kitchen with plumbing.
Second stories. These often need the existing walls and foundation strengthened.
Design, engineering and permits. Every addition needs drawings and structural review.

Permits

Permits for additions in Bellevue

Adding square footage always requires a permit with plan review in Bellevue. Additions over 3,000 square feet of new or replaced floor area fall under the city's new-house or large-addition permit. Setbacks, lot coverage, critical areas and slopes vary parcel by parcel, so we check your property's rules before we quote, then manage design, permitting and inspections from start to finish. For the rest of the Eastside, see what needs a permit; for a second unit, see our ADU guide.

Get a Free Estimate Our additions and ADUs

Questions

Addition questions

Will the price grow once the work starts?

Not because we left things out. Before you sign, you get a detailed written scope that lists what will be done, room by room, with the materials and allowances — so you know exactly what you are paying for. We do not lowball an estimate to win the job and then add a list of extras once the walls are open. The only changes are things nobody could see in advance, such as damage behind a wall; we photograph them, explain them and price them in writing before any extra work starts. The decision is always yours.

How do you protect our home while we live in it?

We treat your home as if we lived there. Before work starts, we put up dust barriers and seal off the work area, cover floors along the path to it and protect furniture and finishes nearby. The crew keeps the site tidy and cleans up at the end of every day, and your project manager walks you through what to expect each week so nothing comes as a surprise.

How long does a home addition take?

A bump-out takes about 1–2 months of construction, a room addition 3–5 months and a second story 6–9 months. Design and permit review come first and depend on the city's workload.

Is it cheaper to add on or to move?

For many Eastside homeowners an addition costs less than the transaction costs of selling and buying — and you keep your neighborhood and schools. We can walk through the numbers with you.

Can we stay in the house during an addition?

Usually yes for ground-floor additions. Second stories are more disruptive, and moving out for part of the project is sometimes easier.

Do you handle design and engineering?

Yes. We coordinate design, structural engineering and permits, or we build from your own architect's plans.

Sources: City of Bellevue — residential building permits. Checked November 2026.