Home Addition Cost in Seattle: Bump-Outs to 2nd Stories
Quick answer: Home additions in the Seattle–Tacoma area typically run $400–$600+ per square foot in 2026, all-in. That means a small bump-out lands around $15,000–$50,000, a ground-level room addition around $150,000–$300,000, a primary suite around $200,000–$400,000, and a full second-story addition $300,000–$600,000+. The right type depends on your lot, your foundation, and your goal — and sometimes the honest answer is that a backyard ADU beats an addition entirely.
Key facts
- Seattle-area additions in 2026: roughly $400–$600+ per sq ft built, permitted, and finished — labor and code costs here sit well above national averages.
- Bump-out (2–6 ft push, ~20–60 sq ft): $15,000–$50,000 — often cantilevered, sometimes no new foundation.
- Room addition (300–500 sq ft family room or bedroom): $150,000–$300,000.
- Second story: the biggest move — new structure over the whole house, roof rebuilt, $300,000–$600,000+ and 6–9+ months.
- Lot rules decide direction: if setbacks and lot coverage are maxed, you build up, not out.
- If the goal is rental income or independent living space, a detached ADU is usually the smarter vehicle than an attached addition.
The four ways to add space — and what each really costs
| Addition type | Typical size | Installed cost (Seattle–Tacoma) | Timeline |
|---|---|---|---|
| Bump-out (kitchen, bath, closet push) | 20–60 sq ft | $15,000–$50,000 | 4–8 weeks |
| Room addition (family room, bedroom) | 300–500 sq ft | $150,000–$300,000 | 3–5 months |
| Primary suite (bed + bath + closet) | 350–600 sq ft | $200,000–$400,000 | 4–6 months |
| Second story (full or partial) | 800–1,500+ sq ft | $300,000–$600,000+ | 6–9+ months |
Why per-square-foot costs run high here: an addition is a small house — foundation or structural work, framing, roofing tied into the existing structure, siding matched, plus the rooms people actually add (kitchens and baths) are the most expensive square footage in any home. Seattle's energy code and permitting standards are also among the strictest in the country.
Bump-out vs full addition: the budget question
A bump-out is the highest-leverage move in remodeling: pushing a wall 3 feet can turn a cramped galley kitchen into one with an island, or fit a real walk-in shower where none existed. Because small cantilevers can often ride on extended joists instead of new foundation, the cost per problem-solved is unbeatable. The rule of thumb we give clients: if the floor plan almost works, bump out; if the house is genuinely short a room, add one; if it's short a floor of rooms, go up.
Going up: what a second story really involves
A second-story addition is the most transformative and the most disruptive project in residential construction: engineering review of the existing foundation and walls, the roof comes off, structure goes up, and the house is weather-protected in stages. Most families move out for part of the build. It's the answer when the lot is maxed out — Seattle setback and lot-coverage limits often make "up" the only direction left — and when you love the location too much to leave. Budget honestly: $300,000–$600,000+, 6–9+ months, and a temporary-housing line item.
Addition or ADU? The question worth $100k
Here's the fork most Seattle homeowners don't know they're standing at. An attached addition grows your home — best for a bigger kitchen, a primary suite, one more bedroom under your roof. A detached ADU (backyard cottage) creates a separate legal dwelling — best for rental income, housing parents or adult kids, or long-term property value with an exit (Washington now even allows ADUs to be sold as condominium units). Costs overlap ($250,000–$450,000 for a DADU), so the decision is about purpose, not price. If your goal sounds like "income" or "independence," start with our ADU & DADU page — our specialist sister company handles those builds end to end.
Permits, timelines, and living through it
Every addition needs permits — structural plans, energy code compliance, and inspections; second stories add engineering review. In Seattle and Tacoma, permitting typically takes 6–12 weeks before construction starts, so realistic calendars read: bump-out ~2–3 months door to door, room addition ~5–7 months, second story ~9–12 months. Ground-level additions usually let you live at home through construction; second stories usually don't for at least part of the build. We handle design, permits, and construction under one fixed-price contract — the number you sign is the number you pay.
Ready to price a home addition in Seattle?
NorthWest Home Remodeling handles additions of every scale across Seattle, Tacoma, Bellevue, and the rest of King and Pierce counties — bump-outs, room additions, primary suites, and full second stories. Text or call +1 (206) 536-8410 for a free on-site estimate, or send us your project details and we will reply the same day.
Frequently asked questions
- How much does a home addition cost per square foot in Seattle?
- Plan on roughly $400–$600+ per square foot all-in for 2026 — design, permits, structure, and finishes. Kitchens and bathrooms land at the top of the range; simple bedroom space at the bottom.
- How much does a bump-out addition cost?
- Typically $15,000–$50,000 depending on size and whether it can cantilever off existing framing or needs foundation work. It's the most cost-effective way to fix a floor plan that almost works.
- Is it cheaper to add up or out?
- Building out is usually cheaper per square foot if your lot allows it — no roof removal, simpler structure. Building up wins when setbacks or lot coverage are maxed, or when you don't want to sacrifice yard.
- Do I need a permit for a home addition?
- Yes — always. Structural plans, energy code, and inspections apply in every Seattle–Tacoma jurisdiction, and second stories add engineering review. We manage permitting as part of the project.
- Should I build an addition or an ADU?
- If the space is for your own household — addition. If the goal is rental income, independent living for family, or a sellable unit, a detached ADU is usually the smarter vehicle; costs are comparable, purposes differ.
- Can I live in my house during an addition?
- Usually yes for bump-outs and ground-level additions (expect dust and staged access). Full second-story additions typically require moving out for part of the build.