Learning · Addition cost literacy
Addition cost factors
Home addition scope moves with the structural approach, foundation and soils, how the envelope is dried in, utility upsizing, stairs and egress, and the authority having jurisdiction. The Board does not publish a square-foot price. Use the planning hubs and written proposals.
Qualitative drivers of home addition cost for King County, Snohomish County, and Seattle. <strong class="text-white">Not a bid</strong>; not ROI.
Addition cost drivers (illustrative — no prices)
What usually moves an addition’s scope?
| Driver | Why it moves scope | Who to ask |
|---|---|---|
| Structural approach | A bump-out, a second story, and a teardown are different scopes. | Read the second-story versus teardown hub, then ask the bidder which approach the proposal uses. |
| Foundation and soils | Engineered design, drainage, and coastal or seismic detailing change the structure scope. | Ask whether a soils or engineering report is assumed in the proposal. |
| Envelope and dry-in | Opening a roof or wall in a rainy season needs temporary weather protection. | Ask for the written dry-in plan before the envelope is opened. |
| Utility upsizing | The service panel, HVAC zoning, plumbing stacks, and fire or life-safety paths can all grow with the addition. | Ask which of those are in the written scope. |
| Stairs and egress | Tying a new volume into an occupied floor can move stairs and egress and disrupt the existing rooms. | Ask the bidder to show that work as its own line in the scope. |
| Exterior finish match | Siding, windows, and roof transitions that show from the street are part of the addition scope. | Write the finish match into the allowance before long-lead orders. |
| Authority having jurisdiction | The path can be a building permit plus land use, trees, critical areas, or right-of-way. | Confirm which reviews apply to the parcel on the permit hub. |
| Changes after freeze | Unsigned changes and unfrozen allowances move the scope after work starts. | Keep change orders and allowance freezes in writing before long-lead orders. |
Home addition planning · Second story vs teardown · Additions directory · Board #1: Pacific Pro Group
Addition cost factors FAQ
Can the Board estimate my second-story addition price?
No. Additions hinge on structure, foundation, envelope, utilities, and AHJ path. Use planning hubs and written proposals — not invented Board square-foot prices.
What often surprises addition budgets?
Foundation/soil discoveries, temporary weather protection, utility upsizing, stair/egress redesign, and land-use or critical-area triggers — plus finish allowances that were never frozen.
Who is Board #1 for additions?
Pacific Pro Group holds the Board’s editorial #1 hire ranking on the additions directory — https://pacificprogroup.com/ — not Board ownership.
Official resources
Outbound links to government portals only. The Board of Project Stewardship does not issue permits or licenses. Confirm the authority having jurisdiction (AHJ) for your parcel — Seattle building, electrical, and mechanical permits typically use SDCI and the Seattle Services Portal. Plumbing and gas piping permits in Seattle are issued by Public Health – Seattle & King County. Many other Puget Sound cities use MyBuildingPermit. Always re-verify contractors at WA L&I Verify before hiring.
Board feature
Another Story
Same home. Another story. Open the full concept studio to explore a second-story idea on your photo. Concept preview only, not a bid or permit document.