Home Addition Cost Estimator

The estimate runs on your own build cost per square foot. A contractor quote, or what a neighbor paid for a similar addition last year, beats any average. That rate should cover framing, roofing, insulation, drywall, windows, electrical, plumbing stub-ins, and finishes. It does not include furniture or landscaping.

If you have nothing local yet, these are rough US figures per square foot, approximate for 2026 and before permits, design and contingency. Real bids often land 30% or more away from them, in either direction.

Low-cost means much of the Midwest and South. High-cost means places like California, New York and Massachusetts.

Quality tiers

Basic means contractor-grade finishes throughout: standard windows, vinyl flooring, builder-grade fixtures. This is the floor for getting a livable, code-compliant space.

Mid-range is where most residential projects land: solid wood or quality laminate cabinets, tile flooring, standard-spec windows, recessed lighting. The gap from basic is visible the moment you walk in.

High-end adds hardwood floors, custom cabinetry, upgraded windows, detailed trim work. Luxury is full custom: specialty tile, premium appliances if it is a kitchen addition, smart home integration.

The costs people forget

Design fees run 8-15% of construction cost. Skip a licensed architect and hire only a draftsperson if you want to save money, but for structural work (a second story, a cantilevered extension) you need a licensed engineer at minimum.

Permits vary by municipality. A straightforward addition in a rural county might cost $800; the same permit in a major city costs $3,000-5,000. Leave the permit box blank and the total leaves permits out, and says so. Your building department can quote the real number in one phone call.

The 12% contingency is not optional. On any project over $50,000, unexpected costs are the rule: a beam where none was expected, old electrical that needs upgrading, moisture damage behind existing walls. Budget for it before the project starts, not after.

Timeline

A 400 sq ft addition typically takes 4-6 months from permit approval to punch list. Permitting itself takes 4-12 weeks depending on the municipality, and it happens before any construction begins.