Adding square footage to your home typically costs $150–$400 per square foot, with bathroom additions on the higher end and garage or sunroom additions on the lower end. Use this free home addition cost calculator to estimate your project based on size, room type, foundation choice, and finish level. Results include a full breakdown across foundation, framing, exterior, interior, and mechanical (HVAC, plumbing, electrical).
Foundation choice and addition type are the biggest variables. A full basement foundation costs 4–5x more than a slab. Bathroom additions cost more per square foot than bedrooms because of plumbing and tile work. Garages are cheaper because they're unfinished. Full-story additions (building up) save on foundation cost but add structural reinforcement to the floor below. Finish level can swing the final number by 30–50%.
This home addition calculator builds your estimate from a base cost per square foot that changes with the room type, then layers on a foundation charge, a finish-level multiplier, and a regional adjustment. Bedroom additions start near $200 per square foot, bathrooms near $320 (plumbing and tile push them up), sunrooms around $180, garages about $85 because they stay unfinished, and a full second-story addition runs about $240. On top of that base, the tool adds a per-square-foot foundation charge — roughly $12 for a slab, $20 for a crawl space, and $55 for a full basement. A full-story addition skips a new foundation but pays about $18 per square foot for floor reinforcement below. Your finish level (basic 0.85×, standard 1.0×, high-end 1.40×) and region (Southeast around 0.92× up to West Coast around 1.22×) scale the whole figure, which is then divided across foundation, framing, exterior, interior, mechanical, and permits.
Say you're adding a 400-square-foot standard-finish bedroom on a crawl-space foundation in the Midwest. The base is $200/sqft plus $20/sqft for the crawl space, or $220/sqft. Multiply by 400 sqft to get $88,000, then apply the standard finish (1.0) and the Midwest factor (0.95) for roughly $83,600 all-in. The breakdown lands near framing $16,700, interior finishes $20,900, mechanical $16,700, and exterior $12,500, with the remainder covering foundation work, permits, and design. Switch to a high-end finish and the same 400-square-foot bedroom climbs past $117,000 — which is why finish level is one of the first decisions worth pinning down.
Building up avoids a new foundation but usually requires reinforcing the walls and floor below, temporarily removing the roof, and staging materials at height. Building out is often simpler structurally but consumes yard space and needs new footings and excavation. For many homes the two approaches land within 10 to 15 percent of each other, so lot size and how the new space ties into the existing floor plan usually decide it.
Almost always. Any addition that changes your home's footprint or adds conditioned living space requires a building permit, and most jurisdictions also require separate electrical, plumbing, and mechanical permits plus inspections. Unpermitted additions can stall a future home sale and may have to be torn out or retroactively approved.
A simple single-room bump-out might take six to eight weeks of construction. A full-story or multi-room addition commonly runs three to six months once you include design, engineering, permitting, and inspections. Weather, material lead times, and change orders are the most common causes of delay.
Yes. Adding permitted, finished square footage increases your home's assessed value, so your property tax bill will typically rise. The exact increase depends on your local assessment rate and how the new space is classified, but most homeowners should budget for higher annual taxes after the project.
A note from me: I've planned dozens of additions, and the numbers here are realistic planning figures, not contractor quotes. Site access, existing structure surprises, and your finish choices will move the real bid — always get two or three local quotes before you commit. — Lisa Tran