Home Addition Cost Calculator

Per-SF cost from foundation, structure, and finish.

Addition sq ft

Structure type (0-3)

Foundation type (0-2)

Finish level (0-2)

Add bathroom (0/1)

Add plumbing (0/1)

Extend HVAC (0/1)

Contingency %

Tab to move between fields

Total addition cost ($)

$76,713.00

≈ 7 years of state college

Cost per sq ft ($)$256.00

Shell / structure ($)$39,000.00

Roofing tie-in ($)$4,000.00

Interior finishing ($)$7,500.00

HVAC extension ($)$3,000.00

Electrical ($)$1,500.00

Permits ($)$1,247.00

Design / engineering ($)$3,117.00

Contingency ($)$10,006.00

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: 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).

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.

Identify Input Parameters
4 parameters
Addition 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 cost
Total addition cost = subtotal + contingency
76713 = $76,713
Calculate Cost per sq ft
256 = $256
Calculate Shell / structure
Shell / structure = additionSF * (basePerSF + foundationAdder)
39000 = $39,000