Home Addition Cost Calculator

This calculator builds a cost estimate for adding onto an existing house, one line at a time. Enter the new floor area and a build rate, then a foundation rate, a tie-in figure for meeting the existing house, any bathroom or kitchen, heating and cooling, and percentages for design and contingency; every line is printed alongside the number it was worked out from. Counted in are the new floor area, its foundation, the tie-in, the rooms and services you list, design and permit fees and a contingency on top of all of it. Left out are financing, any work on the rest of the house beyond the tie-in figure you enter, an electrical service upgrade, asbestos or lead abatement, landscaping, somewhere to live during the work, furniture and sales tax on materials. Additions cost more per square foot than the same space built on an empty lot, so treat the result as a frame for reading contractor quotes rather than a budget.