Free tool · Addition Cost Calculator
How Much Does a Home Addition Cost on the Eastside?
On the Eastside, a ground-floor addition typically costs $400–$650 per square foot with mid-range finishes, a second story $400–$520, and a detached ADU $500–$640, including design, engineering and permits. Small projects cost more per foot. Pick a type, size, finish level and city below for a construction range, permit range and timeline.
{{ typeLabel }} · {{ sqft }} sq ft · {{ finishLabel }} · {{ city }}
Construction cost range
{{ costText }}
{{ psfText }}
Permit and plan review
{{ permitText }}
Design + permit
{{ designWeeks }}
Construction
{{ buildWeeks }}
{{ typeNote }}
Construction range includes design, engineering and contractor costs. Permit range covers building permit and plan review only.
How the addition calculator builds its range
The calculator multiplies your square footage by a cost-per-square-foot range for the addition type and finish level, drawn from Eastside residential projects. It then applies a minimum project cost for small additions, adds fixed site costs where a type always needs them, and adjusts for the city. The permit range uses the same fee model as our Permit Cost and Timeline Estimator, with valuation set to the middle of the construction range.
Assumptions behind the numbers
- Single-family house on a typical lot with reasonable access for equipment.
- Includes design, structural engineering, energy code compliance, permits, construction and contractor overhead.
- Kitchens or bathrooms inside the addition push results toward the top of the range.
- Second stories assume moderate foundation reinforcement; severe foundation deficiencies are outside the range.
- Detached ADUs include a typical utility connection; long sewer runs, pumps or utility upgrades are extra.
- Excludes Washington sales tax, impact fees, utility connection charges, geotechnical reports and landscaping.
| Type | Standard | Mid-range | High-end |
|---|---|---|---|
| Bump-out | $450–$600 | $550–$750 | $700–$1,000 |
| Room addition | $380–$480 | $450–$600 | $580–$800 |
| Primary suite | $420–$520 | $500–$650 | $640–$850 |
| Second story | $330–$420 | $400–$520 | $500–$700 |
| Detached ADU | $420–$520 | $500–$640 | $620–$820 |
| Garage or basement ADU | $230–$300 | $280–$380 | $360–$480 |
Sources and references
- ECD project cost records for Eastside additions and ADUs.
- City fee schedules through MyBuildingPermit.com and Seattle SDCI.
- WSU Energy Program materials on the Washington State Energy Code, which shapes insulation and window costs.
- Washington Department of Commerce guidance on ADUs.
Cost questions
Questions about addition budgets
Why do small additions cost more per square foot?
Every addition needs a foundation, a roof tie-in, an opening into the house, permits, engineering and setup, whatever its size. In a 100-square-foot bump-out those fixed costs are spread over few feet, so the per-foot price is high. Adding another 100 square feet costs much less than the first 100, which is why size decisions deserve a second look.
Is it cheaper to build up or build out?
Per square foot, a second story is often cheaper than a ground-floor addition because it needs no new foundation or roof area beyond what the house already has. But it disrupts the whole house, usually requires moving out and may need foundation reinforcement. A ground-floor addition costs more per foot but lets you live at home and affects less of the existing house.
How accurate is an online addition cost estimate?
It is a planning range, not a bid. It shows whether an idea fits your budget before you pay for design. Site conditions such as slope, soil, access, the existing foundation and utility routes move real costs within or beyond the range. A site visit with an engineer narrows it to a figure you can plan around.
What costs are not included in the calculator?
Washington sales tax, impact fees, utility connection charges, geotechnical or arborist reports, landscaping, furniture and temporary housing are excluded. Unusual foundation repair, long sewer runs and electrical service upgrades are also outside the range. Each is identified and priced separately during feasibility.
Free estimate · Bellevue office
Talk to the engineer before you commit to a plan
A site visit, a scope conversation and a written budget range, at no cost.