Home Addition Cost Calculator
Estimate the cost to build a room addition from its footprint and your material and labor rates per square foot.
How this calculator works
This estimates the cost of a room addition from its footprint. It multiplies the addition’s square footage by a material rate per square foot, and separately by a labor rate per square foot, then adds the two together for a total.
Formula: Total cost = addition area × material rate per sq ft + addition area × labor rate per sq ft.
Worked example
A 20 ft × 15 ft addition (300 sq ft), $150/sq ft material, $150/sq ft labor:
- Addition area: 20 × 15 = 300 sq ft
- Material cost: 300 × $150 = $45,000
- Labor cost: 300 × $150 = $45,000
- Total estimated cost: $90,000
Notes
A room addition typically costs more per square foot than new-home construction, because it involves tying into an existing foundation, roofline, and utilities rather than building on an open, unobstructed footprint. A second-story addition avoids the cost of a new foundation, but it adds structural work below — the existing walls and foundation often need reinforcement to carry the added load.
How to use
Measure the length and width of the addition’s planned footprint in feet, then enter local material and labor rates per square foot from a contractor quote. The calculator multiplies the area by each rate and adds them together for a total project estimate.
Estimates only. Actual cost depends on your region, material choice, and site conditions — get a contractor quote before budgeting.
Frequently asked questions
What should I measure for a home addition estimate?
Measure the length and width of the addition's footprint in feet — the finished floor area being added to the home. Multiply length by width to get the square footage that the material and labor rates are applied to.
Why does a home addition cost more per square foot than new construction?
A room addition involves tying into an existing foundation, roofline, walls, and utilities, which adds complexity that a from-scratch new build doesn't have. Matching existing finishes and structural connections also takes more careful, often slower work than building on an open lot.
Is a second-story addition cheaper than a ground-floor addition?
A second-story addition avoids the cost of a new foundation, which can offset some expense. But it adds structural work below — the existing walls and foundation may need reinforcement to carry the added weight, so overall cost depends on the home's existing structure.
Does this estimate include permits or design fees?
No. This calculator covers only material and labor cost per square foot for construction. Permits, architectural or engineering design fees, site prep, and utility connections are separate costs to add on top of this estimate.
Estimates only. Verify quantities with your supplier before purchasing.