Extension costs
What goes into a 4m × 6m extension budget?
A 24m² footprint is a starting point. Here is what to include before treating a figure as your project budget.
CompareYourBuild editorial · 8 September 2026 · Pilot guide
Start with the right measurements
A rectangular 4m projection by 6m width gives a 24m² footprint. External dimensions are not the same as usable internal floor area. Two storeys, wraparounds and existing extensions need their geometry considered separately; do not assume every project is one new rectangle.
Use a range for early construction costs
Our calculator combines the supplied project details with published benchmarks and supported regional adjustments. It shows its sources and assumptions. Location is one input: access, specification, structure and site conditions can still change a builder’s actual offer.
Build a separate list of other costs
Record design, technical drawings, structural engineering, statutory application charges, building-control or warrant charges, and any justified surveys. Check where professional packages overlap. Record VAT treatment for every figure, rather than adding together amounts on different bases.
Record the choices that drive the specification
A flat or pitched roof, glazing, kitchen, bathroom and finish choices need to be described in the brief. Our early model records roof preference but does not price each roof design separately. Contractor quotes are needed to compare those choices reliably.
Keep unknowns visible
Do not assume ordinary groundworks, drainage arrangements or permissions have been confirmed from a postcode. Keep a separate contingency appropriate to the uncertainty and discuss it with your designer or cost adviser. An unverified council tariff remains ‘to confirm’ in our result.
Move from a budget to comparable offers
Enter your compulsory postcode and address to get the supported local context, save the brief and choose the professional service you need. As drawings and specifications develop, compare quotations against that same information. A preliminary estimate is useful for planning; it is not a builder’s commitment.