How Much Does a Home Addition Cost in Seattle & the Eastside? 2026 Price Guide

2026 home addition cost ranges by project type

- Ground-level room addition (bedroom, family room, home office): $350–$550 per square foot in most of King County. A typical 300–400 sq ft addition lands around $120,000–$200,000 including foundation, roof tie-in, and finishes.

- Primary suite addition with bathroom: $180,000–$300,000+. Plumbing, tile work, and closets push the per-square-foot cost toward the top of the range.

- Second-story addition: $300,000–$600,000+ depending on footprint and structural work. You're building a new floor and a new roof, and the existing structure usually needs reinforcement — but you double livable space without losing yard.

- Kitchen bump-out or dining extension: $90,000–$180,000. Small in square footage but dense in cabinetry, plumbing, and electrical.

- Sunroom or four-season room: $80,000–$160,000 depending on glazing, heating, and foundation type. See our sunroom additions page for options.

What drives addition costs up or down in the Seattle area

Three things move the number more than anything else: structure, site, and wet rooms. Opening or removing load-bearing walls to connect the addition (our structural remodeling crews handle this in-house), sloped or tight lots that complicate excavation, and every bathroom or kitchen you add. Labor and materials in King County remain among the highest in the country in 2026, which is why Seattle-area additions price well above national averages.

Permits and timelines: what to expect in 2026

Additions need full building permits everywhere in Washington. Plan on 2–4 months for design and permitting in most Eastside and South King County cities — Seattle can take longer — then 4–8 months of construction depending on scope. Blue Lake handles architectural plans, engineering, permits, and inspections on one fixed-price contract, whether your project is in Seattle, Bellevue, Renton, or Kent.

Addition vs. DADU: which adds more value?

If the goal is rental income or independent space for family, a detached accessory dwelling unit often beats an addition: it has its own entrance, can be rented long-term, and under Washington's HB 1337 most lots can now host one. A DADU typically runs $250,000–$450,000+ — comparable to a second-story addition — but generates $1,800–$3,000+ per month in rent. Compare options on our ADU construction and DADU builders pages.

How to budget with confidence

Get a fixed price, not an estimate. Cost-plus contracts shift overruns onto you; a fixed-price, fixed-duration contract means the number you sign is the number you pay. Blue Lake is a licensed Washington general contractor (#BLUELLC760C3) building additions, full home remodels, and ADUs across King County. Request a free consultation and we'll give you a realistic number for your lot, your structure, and your scope.