Home Addition Cost Calculator
Per-SF cost from foundation, structure, and finish.
About this calculator
This calculator builds a home addition estimate from a per-square-foot shell cost that varies by structure type and finish level — bump-outs and sunrooms are cheapest since they need less structural work, while second-story additions cost the most because they require structural reinforcement of the existing house below, reflected in the highest base rate table (up to $250/SF at upscale finish). A foundation adder layers on top: slab is treated as included in the base rate, crawlspace adds $8/SF, and a full basement adds $25/SF. From there the calculator itemizes real construction line items scaled to your square footage and finish tier — roofing tie-in (fixed by structure type and adjusted for finish), windows (one estimated per 80 SF at $350-1,200 each), siding sized from an estimated perimeter times wall height, HVAC extension, electrical, and interior finishing (drywall, paint, trim, flooring) — plus optional flat costs for adding a bathroom or plumbing if you flag them.
Permit fees and architecture/engineering costs are calculated as a percentage of the materials subtotal rather than flat numbers, and that percentage is highest for second-story additions specifically because they always require a structural engineer regardless of finish level — a detail the calculator bakes in directly rather than leaving to the finish-level table. A contingency percentage (15% by default, adjustable up to 30%) is then applied to the subtotal to produce the final total cost and cost-per-square-foot. Keep in mind these are national-average unit costs: actual pricing swings with regional labor rates, site access difficulty, whether utilities need to be rerouted, and how closely the addition needs to match the existing home's exterior finishes — get contractor quotes for anything beyond rough budgeting, and don't skip the contingency line, since additions routinely uncover hidden issues (foundation problems, code-driven scope changes) once walls are opened up.
Inputs
Results
Total addition cost ($)
$76,713.00
≈ 7 years of state college
How to Use This Calculator
- Enter the addition square footage and select structure type (stick-built, modular, bump-out, or second story).
- Select foundation type (full basement, crawl space, or slab-on-grade).
- Set finish level and toggle bathroom addition to refine the estimate.
- Review total cost, shell cost, roofing, and finish costs broken out individually.
- Add permit, architectural, and landscaping costs to complete the total project budget.
How the result changes with Addition sq ft
| Addition sq ft | Total addition cost ($) |
|---|---|
| 150 | $41,861.00 |
| 225 | $59,372.00 |
| 450 | $111,100.00 |
| 750 | $179,239.00 |
What each input means
- Addition sq ft
- Square footage of the planned addition.
- Structure type (0-3)
- 0 = Bump-out (<100SF), 1 = Single-story, 2 = Second-story, 3 = Sunroom.
- Foundation type (0-2)
- 0 = Slab on grade, 1 = Crawlspace (+$8/SF), 2 = Full basement (+$25/SF).
- Finish level (0-2)
- 0 = Basic, 1 = Mid-range, 2 = Upscale.
- Add bathroom (0/1)
- 1 = Include a bathroom in the addition.
- Add plumbing (0/1)
- 1 = Include plumbing (laundry, wet bar, etc.).
- Extend HVAC (0/1)
- 1 = Extend existing HVAC or add mini-split.
- Contingency %
- Buffer for unforeseen structural or site issues (15% recommended).
What each result means
- Total addition cost ($)
- All-in cost including permits, design, and contingency.
- Cost per sq ft ($)
- Total cost divided by addition square footage.
- Shell / structure ($)
- Foundation, framing, and exterior shell cost.
- Roofing tie-in ($)
- Cost to tie new roof into existing structure.
- Interior finishing ($)
- Drywall, paint, trim, and flooring.
- HVAC extension ($)
- Cost to heat/cool the addition.
- Electrical ($)
- Wiring, outlets, and lighting.
- Permits ($)
- Estimated building permit fees.
- Design / engineering ($)
- Architectural plans and structural engineering.
- Contingency ($)
- Buffer for unexpected costs.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersAddition sq ft = 300, Structure type (0-3) = 1, Foundation type (0-2) = 0, Finish level (0-2) = 1 = 8 input(s) provided
- Calculate Total addition costTotal addition cost = subtotal + contingency76713 = $76,713
- Calculate Cost per sq ft256 = $256
- Calculate Shell / structureShell / structure = additionSF * (basePerSF + foundationAdder)39000 = $39,000
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does a second-story addition cost more per square foot than a single-story one?
Second-story additions use the highest base-rate table (up to $250/SF at upscale finish, versus $190/SF for single-story) because adding weight above an existing structure requires reinforcing the floor and foundation below — work a ground-level or bump-out addition doesn't need. The calculator also forces second-story projects into an 8% design/engineering rate regardless of finish level, since a structural engineer is always required.
How is the number of windows in the estimate determined?
The calculator assumes one window per 80 square feet of addition, rounded up with a minimum of one, then prices each window from $350 to $1,200 depending on finish level. It's a spacing-based estimate rather than a count you enter directly, so an unusually window-heavy or window-light design won't be reflected precisely.
Why does foundation type add a per-square-foot cost instead of a lump sum?
Slab-on-grade is treated as already included in the base shell rate, while crawlspace adds $8/SF and full basement adds $25/SF, both scaled to your addition's total area — foundation excavation and construction cost scales roughly linearly with footprint, not as a one-time fixed charge.
Why is contingency applied after permits and design costs, not just on construction?
The calculator sums shell/materials cost, permit fees, and design/engineering fees into one subtotal first, then applies the contingency percentage to that entire subtotal — not only the physical construction cost — since unexpected costs (a redesign, a permit resubmission) can arise anywhere in the project, not just in the build itself.
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