Home Addition Cost Calculator
Build an addition midpoint from size, structure type, finishes, foundation, roof work, HVAC, and whether you add a bath — then scale by state. Click any i for detail.
Your addition
Addition size (sq ft)
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Addition size
Finished floor area of the new space (length × width of the footprint). |
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Addition type
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Addition type
Second-story work is usually the priciest structure rate; sunrooms are lighter. |
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Interior finishes
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Interior finishes
Flooring, trim, paint, and similar finish work on top of structure. |
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Foundation
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Foundation
New slab is common for bump-outs; basement under the addition costs much more. |
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Roof work
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Roof work
Matching the existing roof is simpler than dormers or complex intersections. |
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HVAC
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HVAC
Extending ducts is cheaper than adding a new zone or dedicated mini-split. |
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New bathroom
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New bathroom
Adding plumbing is a major budget line — powder vs full bath. |
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Design / drawings
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Design / drawings
Basic plans cover permit drawings; full design/engineering is a larger allowance. |
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Your state
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Your state
Scales midpoints vs U.S. average — not a builder bid. |
How to use this calculator
- Enter finished square footage.
- Choose addition type and finish level.
- Set foundation, roof, HVAC, and bath plans.
- Pick design-drawing depth and your state.
- Read the itemized total (permits are included as ~4%).
Results explained
The headline is an itemized midpoint: structure, finishes, foundation, roof, HVAC, bath, design allowance, and permits — after state scaling.
Quick reference: addition building blocks
U.S.-average midpoints before state scaling.
| Line | Midpoint |
|---|---|
| Bedroom / flex structure | $95 / sq ft |
| Second-story structure | $145 / sq ft |
| Mid-range finishes | $95 / sq ft |
| New slab | $18 / sq ft |
| Basement under addition | $55 / sq ft |
| Extend HVAC | $3,500 |
| Add full bath | $22,000 |
Design ~3% or ~6% of subtotal when selected. Permits ~4% of running total.
How the estimate is built
Square footage multiplies structure, finish, foundation, and roof rates for your choices. HVAC and bath add flat packages. Design and permit percentages apply to the running subtotal, then state scaling is applied to each line.
Example scenario
A 400 sq ft bedroom addition with mid finishes, new slab, matching roof, extended HVAC, no new bath, and basic plans at U.S. average pricing typically lands near the low-to-mid $100,000s in this model before second-story or full-bath upgrades.
FAQ
Bump-out vs second story?
Choose second-story addition for the higher structure rate — stairs and engineering usually cost more.
Is kitchen expansion included?
Not as a full kitchen package. Model structure/finishes here and use the kitchen remodel calculator for cabinet/counter scope.
What about connecting to the old house?
Opening walls and matching floors/roof is partly in structure/roof lines; severe structural tie-ins can exceed midpoints.