Seattle, WA · King County
Home Addition in Seattle, WA: Cost & Permits
What home addition actually costs in Seattle, whether you need a permit, which inspections apply, and how long the job takes.
Verified August 2026
Typical local cost
$146,900
$63,000 – $391,700
per sq ft of added space
$250 – $650
Permit fee
$1,080 – $8,100
Modeled allowance
vs national
+40%
local labor & materials
Looking for pricing detail? The full home addition cost breakdown for Seattle covers cost by size and material, the local cost model and nearby markets.
Do you need a permit for home addition in Seattle, WA?
Yes
Yes — a permit is required for home addition in Seattle, WA. The work must be inspected before it can be closed out, and a licensed contractor generally has to pull the permit.
SDCI review timelines, seismic detailing and energy-code requirements add meaningful cost and schedule to structural and envelope work.
Cost breakdown in Seattle
| Low endSimple scope, competitive bid | $63,000 |
|---|---|
| TypicalMost homeowners land here | $146,900 |
| High endComplex access, premium materials | $391,700 |
| per sq ft of added space | $250 – $650 |
| Permit fee | Estimated allowance $1,080–$8,100Modeled from comparable fee schedules — not an official quoted fee. |
| Permit authority | City of Seattle building departmentModeled estimate |
| Timeline | 3-6 months of construction, 6-14 months including design and permitting |
Math: national baseline $45,000–$280,000 × Seattle cost index 1.26. Permit fees use a 1.35 local multiplier. Estimates, not quotes.
What drives your price here
- Added square footage and whether it is one or two stories
- Foundation type and site conditions
- Extending or adding HVAC capacity
- Zoning setbacks, lot coverage and impact fees
- Flood zone elevation requirements
Permit process and inspections
| Permit required | Yes |
|---|---|
| Who pulls it | Licensed contractor |
| Inspections | Foundation / slab inspection, Framing and sheathing, Electrical, plumbing and mechanical rough, Insulation and energy, Final building inspection |
| Documents | Building permit application, Sealed architectural and structural drawings, Site plan and survey showing setbacks, Energy code compliance forms, Notice of Commencement recorded with the county, Contractor license and insurance |
| Apply | Seattle permit portal |
Frequently asked questions
How long does an addition permit take in Florida?
Plan review for an addition typically runs four to twelve weeks depending on the jurisdiction, and that is after the sealed drawings are complete. Coastal and flood-zone properties take longer because of elevation review.
Do impact fees apply to a home addition?
Many Florida counties assess impact fees on added conditioned square footage. The fee is usually calculated per square foot and can add thousands to a large addition, so ask for the fee estimate before design.
Addition costs in nearby cities
Sources
Sources & verification
Last verified August 2026. Cost figures are modeled estimates based on national baselines adjusted for local labor and material pricing — they are not quotes. Permit rules change; confirm with the building department before starting work.
Page data quality: 100/100
- ✓ Verified permit status
- ✓ Official source linked
- ✓ Local cost range
- ✓ Inspection detail
- ✓ Unique local rule
- ✓ Local fee estimate
- ✓ Useful FAQ
- ✓ Relevant calculator
- ✓ Internal links