How We Calculate Home Addition Costs
Free calculators and state-by-state guides that give you a realistic price range before you talk to a contractor. Here is exactly how the numbers are built, and what they can’t tell you.
How an Estimate Is Built
Every calculator on the site uses the same four steps, so a bedroom priced in the room calculator and a room addition priced in the main calculator start from the same numbers.
1. Base Rate
A national cost per square foot for the type of space you are adding. “Wet” rooms such as kitchens and bathrooms carry higher rates than bedrooms because of plumbing, venting and waterproofing.
2. Finish Level
Economy, standard or premium finishes scale the base rate up or down to reflect materials, fixtures and trim quality.
3. State Cost Index
Your ZIP code is matched to its state, and the estimate is adjusted by that state’s cost index, the same index behind our state-by-state guides. It reflects statewide averages, not your specific city.
4. Add-Ons & Range
Items you select, such as a foundation type, a bathroom or a new HVAC system, are added at a national price adjusted by the same state index. The result is shown as a range, from 10% below to 20% above the calculated figure, because real bids vary.
What Our Estimates Can’t Tell You
An online calculator is a planning tool, not a quote. Use it to set a realistic budget and to sanity-check bids, then rely on licensed local professionals for the real number.
City-Level Differences
Costs inside a state can vary by 50% or more between a major metro and a rural county. Our state guides show city ranges; the calculators use statewide averages.
Your Specific Site
Slopes, poor soil, flood zones, old wiring or hidden structural problems can add significantly to a project and only show up on a site visit.
Local Permits and Fees
Permit, impact and utility fees are set locally and change often. Check your building department’s current fee schedule.
Today’s Market
Material prices and contractor availability move. Budget a 15–20% contingency on top of any estimate, and get at least three itemized bids.
How Our Content Is Reviewed
Our cost guides and state pages are reviewed by Tim Martin before they are published and whenever the figures change. Reviews check that ranges are consistent across the site, that the calculators and the written guides use the same numbers, and that permit and licensing statements are worded carefully.
Spotted something out of date or wrong for your area? Tell us and we will check it.
How We Use Your Information
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Estimates are free and need no account You can use every calculator on the page without signing up. Contact details are only requested when you ask for a detailed estimate.
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You decide whether to be contacted When you submit a calculator form, we use your details to send your estimate and, as each form explains, to contact you about your project or connect you with local contractors for quotes.
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The details are in our policy Our Privacy Policy explains what we collect, how it is used and how to reach us.
Featured Resources
Use these tools and guides to plan and execute your home addition with confidence.
Home Addition Checklist
A comprehensive step-by-step guide to planning your project from initial concept to final inspection.
Get the Checklist →Cost by State
City-by-city ranges, permit rules and licensing requirements for all 50 states.
Browse States →Building Permit Guide
Navigate the permit process, understand local requirements, and avoid costly compliance mistakes.
Learn More →