Home Addition Cost Calculator: Itemized Budget

How to estimate a home addition

1. Measure the rectangular footprint and enter the number of stories. Split irregular additions into rectangles and run them separately when needed.
2. Enter wall height and exterior opening area so the exterior-finish quantity is not based on gross wall area alone.
3. Use a roof area factor of 1 for a flat plan area, or a larger factor supplied by the roof design or measured takeoff.
4. Enter project-specific rates for foundation, structure, roofing, exterior finish and interior finish. Add rooms, building systems, design, permits and tie-in work separately.
5. Apply overhead and contingency only when they are not already included in the entered rates or fixed quotes.

Checked example: a 20 × 16 ft, one-story addition has a 320 ft² footprint and floor area. With 9 ft walls, 80 ft² of openings and a 1.12 roof factor, the net exterior wall area is 568 ft² and roof area is 358.4 ft². The sample entries produce a subtotal before overhead of $158,500, a base including 10% overhead of $174,350, and a total with 10% contingency of $191,785.

Formulas used

Footprint = length × width. Total floor area = footprint × stories. Gross exterior wall area = 2 × (length + width) × wall height × stories. Net exterior wall area = gross wall area − openings. Roof area = footprint × roof area factor. Each measured cost equals its applicable area × entered rate. Bathroom cost = bathroom count × average cost. Subtotal before overhead = measured costs + fixed costs. Overhead = subtotal × overhead percentage. Base = subtotal + overhead. Contingency = base × contingency percentage. Total = base + contingency.

Avoid double counting

Clarify whether shell, interior and room quotes already include labor, contractor markup, windows, mechanical work or permits. Set duplicate fields to zero. A second-story addition can require structural reinforcement and temporary protection not represented by a simple floor-area rate; enter those confirmed costs under demolition/tie-in or other costs.

Important limits

This calculator performs arithmetic on user-entered dimensions, rates and costs. It does not infer local prices, soil or foundation design, structural reinforcement, setbacks, zoning, fire separation, energy code, utility capacity, hazardous materials, occupied-home protection, financing, taxes, schedule, escalation or permit approval. The rectangular wall and roof models are planning approximations; use drawings, takeoffs and qualified local bids for decisions.

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