Extension Cost And Value Calculator

Work out extension cost and value instantly with clear inputs, formula shown and shareable results.

About Extension Cost And Value Calculator

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.

Formula

Extension economics

Total cost = area x build rate x (1 + fees); value added = area x local value rate

Worked example

With the default values below, Extension Cost And Value Calculator returns Net value added of $1,850,000.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Extension areaminimum 0.1 ft²: 500 ft²
Build costminimum 0 per ft²: 2,500 per ft²
Local value rateminimum 0 per ft²: 6,500 per ft²
Professional fees and approvalsaccepts 0–30 %: 12 %

Results

Net value added: $1,850,000.00
Total cost including fees: $1,400,000.00
Gross value added: $3,250,000.00
Return on investment: 132.14% %

Step by step

1Values used
2Extension economics
3Net value added= 1,850,000.00
4Total cost including fees= 1,400,000.00
5Gross value added= 3,250,000.00
6Return on investment= 132.14 %

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Extension Cost And Value figure worked out?

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.

Is there a ceiling on what an extension can add?

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.

What values do I need to enter?

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.