Frequently Asked Questions
How does the calculator work out my estimate?
It multiplies a base cost per square foot for your addition type by the size you enter, scales that for the finish level, adds foundation and features at fixed costs, then adjusts by a state cost index.
How does my state change the estimate?
Each state has a cost index relative to the national average, from 0.82 in Mississippi to 1.65 in Hawaii. The calculator applies the index you select.
Why is my estimate a range?
Contractor pricing and material prices vary. The range runs 10% below to 20% above the central figure.
What does the estimate not include?
Design and permit fees, site surprises, landscaping, and furniture are not itemised. Keep a 10–20% contingency.
Which calculator should I use?
Use the main calculator for most additions, and the specialised calculators for rooms, second stories, ADUs, bathrooms, and return on investment.
How accurate is the calculator?
It is a preliminary planning range based on 2026 cost data. Get itemised local bids before building.
Can I use the estimate to compare bids?
Yes, as a sanity check. Compare itemised bids line by line, not only on total price.