Free estimate — verify against local code before building
Home Addition Cost Calculator
Create a low, expected, and high home addition planning budget from visible editable scope assumptions.
What this calculator includes
This early-stage addition estimator separates shell, foundation, interior finishes, mechanical/electrical/plumbing systems, bathrooms, kitchen work, design and permits, site and utility work, and contingency. All rate bands and fixed allowances remain visible and editable because a concept budget cannot substitute for plans, site investigation, or comparable contractor proposals.
Next step in your project
Turn the concept range into a complete project price
Build a proposal-level model for labor, equipment, overhead, contingency, and margin after the scopes and takeoffs are defined.
Open Contractor Job Pricing Calculator →How to use this home addition cost calculator
- 01
Define total floor area
Use conditioned floor area across all proposed stories; the calculator derives an approximate footprint for the foundation allowance.
- 02
Choose visible structural assumptions
Select a foundation planning type and complexity factor, then replace shell and foundation rate bands with concept-estimate information.
- 03
Add rooms and systems
Enter bathrooms, kitchen scope, interior finish, and system rates without assuming they are included in a generic square-foot amount.
- 04
Price the unresolved work
Use design, permit, site, utility, and contingency allowances openly, then replace them as surveys, plans, investigations, and proposals arrive.
Worked example
Example: 600-square-foot one-story addition
A 600 sq ft addition on a crawlspace with one bathroom, no kitchen, editable shell/foundation/finish/system rates, design and site allowances, a moderate complexity factor, and 15% contingency produces traceable low, expected, and high totals plus cost-per-square-foot scenarios.
Practical buying and overage guidance
Before comparing proposals, assemble the same survey, drawings, specifications, allowance schedule, site logistics, utility scope, testing, permit responsibilities, schedule, exclusions, warranty, concealed-condition terms, and change-order process for every bidder.
Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- U.S. Census Bureau - Characteristics of New Housing (opens in a new tab)
National construction data does not determine a local addition price; rates remain user-editable.
- U.S. Bureau of Labor Statistics - Occupational Employment and Wage Statistics (opens in a new tab)
Trade labor conditions vary by location and project.
- EPA Renovation, Repair and Painting Program (opens in a new tab)
Lead-safe requirements may affect additions tied into pre-1978 homes.
Frequently asked questions
Is cost per square foot enough to budget an addition?
No. It can frame an early conversation, but foundation, tie-in, roof, systems, kitchens, bathrooms, site access, utilities, design, and permit scope can dominate the total.
Does this include architect and engineering fees?
Only the editable design and permit allowance is included. Required survey, architecture, engineering, energy, geotechnical, testing, utility, and review services vary by project.
Why is footprint different from total addition area?
A two-story addition can have twice the floor area of its approximate foundation footprint. The formula divides entered total floor area by stories for the foundation planning line.
Does this verify zoning or structural feasibility?
No. Setbacks, lot coverage, easements, utilities, soils, loads, foundations, connections, egress, energy, fire separation, and permits require site-specific professional review.
Are the default rates local prices?
No. They are broad editable scenario inputs, not measured local pricing. Replace them with a qualified concept estimate and comparable written proposals.