Home Addition Cost Calculator

About $73,200 at national-average rates for a mid-finish 400 sq ft bump-out with a slab foundation, HVAC tie-in, drafting fees and permit — which lands just past Angi's $20,900–$72,600 band for typical room additions and well past HomeGuide's note that 20×20 additions start around $50,000. A three-car-garage-deep 600 sq ft suite runs near $105,800.

Building out is far cheaper per square foot: Angi's 2026 data puts ground-floor additions at $80–$200 per sq ft against $300–$500 per sq ft to build up, because going up means tearing off and re-building the roof over the footprint, reinforcing existing walls for the new loads, and adding a staircase. A 200 sq ft second-story addition prices at about $78,060 here versus $40,600 for the same 200 sq ft on the ground — going up only wins when the lot can't take a build-out.

Yes — the shell, slab, roof and walls already exist, so a 400 sq ft two-car conversion runs about $26,290 mid-finish versus $73,200 for the same size as new construction (HomeAdvisor pegs professional conversions at $6,003–$27,348). The budget still has to cover insulation, drywall, an egress window or door, extending HVAC and raising the floor if the slab sits below the house's finished floor — plus a permit, since it's a change of use.

For a 200 sq ft room, our model prices a basic three-season build (no duct extension) around $25,240 and a four-season room with a mini-split around $65,240 — bracketing HomeGuide's published 3-season and 4-season ranges. The jump buys thermally broken glazing, insulated walls and conditioning. It matters at appraisal too: most appraisers don't count a three-season room in finished living area, so a four-season build protects resale value.

Not always. Many jurisdictions accept drafted plans for a straightforward ground-floor bump-out — our model carries drafting at $2,500. You'll want an architect or structural engineer when the addition changes the structure: any second story, removing bearing walls, or unusual foundations (published architect fees run 5–20% of construction; we model 8%, about $4,800 on a $60,000 build). Your permit office will tell you what stamp your plan set needs.

Budget 4–12 weeks for design and permitting before anyone digs — longer in cities with slow plan review — then 6–16 weeks of construction. Real money leaves during the design window, before a shovel moves: drafting runs $2,500 (or 8% of construction for an architect) and the building permit $1,200 on a typical bump-out. Ground-floor bump-outs and garage conversions sit at the low end of the build window; second stories run the high end because the roof comes off mid-project and the weather owns your schedule. Foundation cure time (about a week for a slab) sits at the front of that window.

In most states, yes — a permitted addition adds to your assessed value, and the assessor often picks it up from the permit filing, so the increase can show up within a year or two. The arithmetic is simple enough to preview: on this calculator's 400 sq ft default (about $73,200 of added value), every 0.25% of your local effective property-tax rate works out to roughly $183 more per year — multiply by your county's rate. The bump varies enormously by state (some cap assessment increases until you sell). Ask your local assessor's office for the reassessment practice before you budget, and note some cities also charge impact or utility connection fees on new floor area that this estimate doesn't include.

Electrical panel upgrades or a second HVAC system, septic or sewer capacity upgrades, driveway cuts and landscaping repair after excavation, impact and connection fees, furnishings, and temporary housing if the work displaces your household. Lead-paint or asbestos abatement in pre-1980 homes is also excluded — that's its own licensed trade.