INTERIOR
Home Addition Estimate Checklist
Contractor checklist for estimating home additions: area, floors, foundation, structure, MEP and contingency.
Home addition bids get eaten by three things: the foundation, the MEP rough-in and the contingency nobody planned. This checklist keeps the estimate complete.
THE SHORT ANSWER
What should a home addition estimate include?
Measure the footprint, price foundation, structure, MEP and finish as separate line items, add design and permits, then 10-15% contingency. A 450 sq ft addition runs $50,000-$90,000+
Measure the footprint and count floors
Measure the actual addition footprint in sq ft and confirm how many stories. A second story adds 15% on top of the same footprint because of structure and staging.
Price foundation and structure
Slab is the cheapest foundation; full basement is the most expensive. Wood frame is standard; steel and ICF add cost. These line items often exceed the finish budget.
Budget MEP and finish separately
Plumbing, electrical and HVAC rough-in runs $15-$40/sq ft. Finish level runs $120/sq ft budget to $220/sq ft premium. Design and permits are real line items, not overhead.
Add the contingency and check
Add 10-15% contingency — additions on existing homes always find surprises. Use the Home Addition Cost Calculator and stay within 10% before markup.