Additions and Remodeling in Seattle, WA
Ground-floor additions in Seattle cost $375 to $600 per square foot and second stories $425 to $675, at the top of regional ranges because old foundations usually need work first and tight lots slow construction. Seattle also has a remodel that is rare elsewhere: digging out a basement to gain full headroom.
Seattle addition and remodel costs
| Project | Typical Seattle range | Seattle factors |
|---|---|---|
| Basement dig-out and finish | $150,000 – $350,000 | Underpinning, new slab, drainage, egress |
| Attic conversion with dormers | $120,000 – $250,000 | Stair, dormer framing, energy code |
| Rear ground-floor addition | $375 – $600 per sq ft | Narrow lots, foundation tie-in |
| Second story | $425 – $675 per sq ft | Foundation upgrades, seismic, height limits |
| Whole-house remodel of a bungalow or Tudor | $300,000 – $750,000 | Systems replacement, structural repair |
Seattle's three ways to gain space
Down. Many Seattle houses sit on basements with seven feet or less of headroom. Lowering the floor means underpinning the foundation in sections, pouring a new slab, adding drainage and egress windows. It is expensive but adds space without touching the roof or yard.
Up. Attic conversions with shed or gable dormers suit Craftsman houses with steep roofs. Full second stories suit ramblers and bungalows with low roofs, but must fit the zone's height limit and the foundation's capacity.
Out. Rear additions are limited by setbacks and lot coverage on narrow lots, and by trees protected under the city's tree rules.
Old foundations and seismic upgrades
Most Seattle houses built before 1950 are not bolted to their foundations and have unbraced cripple walls. Any addition or second story is the moment to bolt and brace, because the engineer will review the foundation anyway. Some early foundations are unreinforced concrete or brick and may need replacement before carrying more load.
Seattle permits for additions and remodels
Additions, second stories, dig-outs and structural remodels go through SDCI's full plan review with structural drawings and energy compliance. Lots in environmentally critical areas need geotechnical input. Unpermitted work by earlier owners may need to be inspected and brought into the new permit, which is worth assessing before you submit.
Seattle addition and remodel questions
How deep can I dig a Seattle basement?
Usually enough for eight feet of headroom, limited by the foundation, the side sewer elevation and groundwater. An engineer sets the method.
Will my second story block a neighbor's view?
Private views are not generally protected in Seattle's single-family zones, but height limits and setbacks still apply.
What happens to unpermitted work found during my remodel?
SDCI may require it to be inspected or corrected within your permit. Assess it early so it is in the budget.
Can I live in the house during a dig-out?
Rarely. Most families move out for the structural phase.