Home Addition Cost Calculator
Estimate renovation budgets with adjustable project dimensions, rates and contingency. All prices are illustrative USD inputs, not local contractor quotes.
Estimate project cost
Budget planning only. Local labor, permits, finishes and site conditions can materially change actual costs.
Define the new floor area and construction scope
- Enter the planned addition area in square feet, excluding unchanged existing floor area.
- Enter a base rate matching the addition type and finish level, then set contingency.
- Enter design and permit costs only if excluded from the base rate; calculate the total budget.
Home Addition Cost Calculator: formula and assumptions
Budget = addition area × base cost per ft² × (1 + contingency ÷ 100) + design and permits.
Worked calculation
400 ft² × $250 × 1.15 + $12,000 = $127,000.
What this estimate includes
Design and permits are added after contingency. The base rate must cover the agreed construction scope; foundation work, utilities and upgrades can vary substantially. This is an illustrative budget, not a contractor quote.
Planning details that change the estimate
A ground-floor extension, second-story addition and garage conversion do not share the same structural and utility requirements. Review foundations, connection to the existing building, architectural work, utilities, HVAC and temporary protection with the project team. The formula adds design and permits after applying contingency to the area-based cost, so avoid including those fees in both places.
Home Addition Cost Calculator FAQs
Is a second story priced the same as an extension?
Not necessarily. Enter a rate based on the actual construction type and scope.
Does contingency apply to design fees?
No. The design-and-permits amount is added after the contingency calculation.
Does this include every project fee?
Only costs represented by your base rate and fixed allowance. Confirm exclusions in the written scope.
Continue planning this project
Estimate separate material quantities with the Concrete Footing Calculator and Insulation Calculator. Use the Roof Replacement Cost Calculator for a roofing scope check, avoiding overlap with the addition’s base rate.
RESULTS
Addition area: 400 ft²
Base cost: $250/ft²
Area subtotal: $100,000.00
Contingency (15%): $15,000.00
Design and permits: $12,000.00
Estimated total: $127,000.00
Additional worked example and planning table
Illustrative base-rate scenarios for 400 ft², 15% contingency, $12,000 design and permits (not a verified regional quote)
Prices vary by region, addition type, structure, finishes, and local requirements.
| Illustrative base rate | Area subtotal | Total with contingency and design/permits |
|---|---|---|
| $150 per ft² | $60,000 | $81,000 |
| $250 per ft² | $100,000 | $127,000 |
| $350 per ft² | $140,000 | $173,000 |
Check: $60,000 x 1.15 + $12,000 = $81,000; $100,000 x 1.15 + $12,000 = $127,000; $140,000 x 1.15 + $12,000 = $173,000.
Second worked example: 300 ft², $200 per ft², 10% contingency, $8,000 design and permits
Step 1: Area subtotal = 300 x $200 = $60,000.
Step 2: Contingency = $60,000 x 0.10 = $6,000, so subtotal with contingency = $66,000.
Step 3: Add design and permits after contingency: $66,000 + $8,000 = $74,000.
Result: $74,000 estimated total.
More frequently asked questions
How do I measure a multi-room addition? Measure each new room as length by width, add the room areas together, and exclude unchanged existing floor area from the addition area.
What if design and permits are already included in the base rate? Enter $0 for design and permits, so they are not added a second time. Check the scope list in the quote first.
Can I price a phased addition with this tool? Yes, in planning terms: run each phase separately with its own area, rate, contingency, and design/permits, then add the phase totals. Keep shared work, such as foundations, in only one phase.
Sources and method note
Method: budget = addition area x base cost per ft² x (1 + contingency / 100) + design and permits, with design and permits added after contingency. No named source is cited here. Prices vary by region, addition type, structure, finishes, labor, and local requirements, so treat the result as a planning estimate only.
Related calculators
- Kitchen Remodel Cost Calculator – Estimate kitchen work inside or alongside an addition.
- Bathroom Remodel Cost Calculator – Estimate a bathroom if one is included in the addition.
- Roof Replacement Cost Calculator – Check roofing scope separately to avoid overlap with the addition base rate.