Work out extension cost and value instantly with clear inputs, formula shown and shareable results.
An extension pays when the local value rate per square foot exceeds the all-in build cost per square foot. The gap is the profit, so the same extension can be strongly profitable in a high value area and loss-making in a low value one, with build cost varying far less between locations than value does.
Extension economics
Total cost = area x build rate x (1 + fees); value added = area x local value rate
Total cost = area x build rate x (1 + fees); value added = area x local value rate. An extension pays when the local value rate per square foot exceeds the all-in build cost per square foot.
Yes, the street ceiling. Once the house is the most valuable on the road, additional area recoups poorly no matter what the average rate per square foot suggests.
This calculator takes 4 inputs: Extension area, Build cost, Local value rate, Professional fees and approvals. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.