Home Addition Cost Calculator

Estimate the cost of adding square footage to your home based on size and addition type.

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Enter your values and click Calculate

A home addition is one of the most significant investments a homeowner can make, permanently increasing both usable living space and the property's market value. Unlike remodels that work within the existing footprint, additions require foundation work, new exterior walls and roofing, and full integration with the home's existing systems — plumbing, electrical, and HVAC. Room additions — a new bedroom, family room, home office, or sunroom — are the most common type and require full residential-grade construction throughout, including insulation, drywall, windows, and finished flooring. Garage additions are significantly cheaper per square foot because they require less insulation, no finished interior walls or flooring, and simpler mechanical connections. Sunrooms and enclosed porches occupy a middle ground, needing foundations and weatherproofing but typically simpler HVAC requirements if designed as three-season spaces. Second-story additions are the most structurally complex because they require engineering assessment and often reinforcement of the existing first-floor framing and foundation before any vertical construction can begin. In-law suites, also called accessory dwelling units (ADUs), must meet full livability standards — dedicated kitchen, bathroom, bedroom, and private access — making them nearly as expensive per square foot as a standard room addition but often subject to additional zoning and permitting requirements. All cost estimates include design, permits, foundation, framing, exterior finishes, and mechanical rough-in. Interior finish packages, appliances, and site-specific conditions are priced separately by most contractors.

How It Works

Cost is estimated by multiplying the addition's square footage by the per-square-foot rate for the selected addition type: garage at $85/sq ft, sunroom/enclosed porch at $140/sq ft, room addition at $175/sq ft, in-law suite at $200/sq ft, and second-story addition at $250/sq ft. These rates reflect fully completed additions including permits, design, foundation, framing, exterior finishes, and standard mechanical rough-in. The low estimate is 75% of the midpoint; the high is 135%. Home additions frequently encounter cost overruns from unforeseen structural issues, utility relocations, or permit delays. Building a 10–15% contingency into your budget is strongly recommended by contractors and financial advisors alike. Getting multiple quotes and a detailed written scope from each contractor is essential.

Examples

400 sq ft Room Addition — New Master Bedroom Suite
Adding a 400 sq ft master bedroom suite with an en-suite bath to a single-story home.
Result: Estimated range: $52,500 – $94,500. At $175/sq ft, a 400 sq ft room addition has a $70,000 midpoint.
600 sq ft Two-Car Garage Addition
Adding a detached two-car garage with a standard concrete floor to a suburban home.
Result: Estimated range: $38,250 – $68,850. Garage additions at $85/sq ft give a $51,000 midpoint for 600 sq ft.

Frequently Asked Questions

Do home additions require permits?
Yes — virtually all home additions require building permits, and most also require architectural plans and multiple inspections (foundation, framing, rough mechanical, final). The permitting process timeline varies by municipality from a few days to several months in high-demand areas. Always confirm permit requirements and estimated timeline with your contractor before signing a contract. Unpermitted additions can create significant problems when selling your home, obtaining financing, or making future insurance claims.
Does a home addition increase property taxes?
Adding square footage typically increases your home's assessed value, which can raise annual property taxes. The increase varies significantly by jurisdiction — some counties reassess immediately after permitted work, others reassess on a fixed schedule. In California, Proposition 13 limits reassessment to the added value only. In many states, the addition's value is simply folded into the next regular assessment cycle. Check with your local tax assessor's office for an estimate of the likely tax impact before committing to a major addition.

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