Why additions cost more per square foot
The per-foot premium comes from scope you don't have on a clean new build:
- Tie-in work — cutting into existing walls, roof, and foundation and matching what's there
- Small scale — fixed costs (permits, mobilization, a crew showing up) spread over fewer feet
- Matching existing — siding, roofing, and finishes that must blend with the current home
- Disruption & protection — working around a lived-in house
Typical ranges by addition type
As 2026 Southeast planning ranges:
- Bump-out / small room: ~$25,000 – $60,000
- Primary suite or family room (300–500 sq ft): ~$70,000 – $150,000
- Second-story addition: ~$150,000 – $300,000+
- Detached garage / ADU: varies widely — budget from real plans
How to budget your addition accurately
Because so much of an addition's cost is the connection to the existing building, generic calculators miss badly. The right approach: price the new conditioned area as new construction, then add the tie-in scope (demo at the joint, cut-ins, structural connection, matching).
Build 360 does exactly this — upload your addition plans and it produces an itemized budget that prices the added area and the tie-in, with a material takeoff and schedule.