Home Addition Cost Calculator
Planning a home addition starts with a realistic budget. This calculator builds one from your own numbers: the square footage you are adding, your cost per square foot from a local quote, plus design fees, permits, and a contingency percentage for surprises. It returns the base build cost, the contingency amount, and a grand total, along with an all-in cost per square foot. Because construction prices vary widely by region and finish, no figure is hardcoded; you supply each one, so the estimate matches your actual project.
Home addition cost formula
Base build cost = square footage * cost per square foot
Subtotal = base build cost + design fees + permit fees
Contingency amount = subtotal * contingency % / 100
Total = subtotal + contingency amount
All-in cost per sq ft = total / square footage
Contingency is applied to the full subtotal including soft costs. The all-in cost per square foot divides the grand total by the added area, giving a single comparable rate.
Home addition budgeting context
- Cost per square foot depends on region, finish level, and whether you add up or out.
- Soft costs such as design and permits are separate from the build cost per square foot.
- A contingency of a set percentage protects against hidden conditions and change orders.
- Permit fees are set by your local jurisdiction; check your building department.
- Get multiple contractor quotes to set a realistic cost per square foot for your area.
Home addition cost: frequently asked questions
How do I estimate the cost of a home addition?
Multiply the added square footage by your local cost per square foot to get the base build cost, then add design fees and permits, and apply a contingency percentage for the unexpected. This calculator sums all four to give a total and a cost per square foot including extras.
What cost per square foot should I use?
Cost per square foot varies enormously by region, finish level, and project type, so there is no single true figure to hardcode. Use a quote from a local contractor or your own budget figure. The default is only a placeholder to be replaced with your real number.
Why add a contingency?
Renovations commonly encounter surprises: hidden structural issues, code upgrades, or material price changes. A contingency, often a set percentage of the build cost, protects your budget. You choose the percentage that fits your risk tolerance and project complexity.
Are permits and design fees included?
They are separate inputs you enter directly, because permit fees are set by your local jurisdiction and design fees depend on your architect or designer. Keeping them as inputs avoids guessing figures that vary by location and scope.
Does this calculator hardcode any prices?
No. Every monetary figure is an input you provide. The calculator only performs arithmetic on your numbers, so the estimate reflects your actual quotes and assumptions rather than an assumed national average.
Official sources
- U.S. Census Bureau: Construction Spending data.
- U.S. Bureau of Labor Statistics: Producer Price Index for construction.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.