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Home Extension Cost Calculator

Cost an extension including the two things that separate it from new build: the structural opening, and making good the house you already have.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-27 · v1.0.0

Market
Imperial · sales tax

Total extension cost

0 currency

Check your inputs

This calculator contains no market rates. Every rate is yours to supply, because a published national average is wrong for almost every individual project and budgeting against one is how projects run out of money.

Extension shell and fit-out
0 currency
Structural opening
0 currency
Making good the existing house
0 currency
Professional fees
0 currency
Contingency
0 currency
Tax
0 currency
All-in cost per m²
0 currency/m²
Non-shell share of the total
0 %

With the figures above, the total extension cost comes to 0 currency. Note that extension floor area and build rate (per m²) sit at the edge of the range this calculator was checked against, so treat the output as indicative rather than settled. The method behind this is well established, though site conditions and material batches will move it somewhat. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Element-based build-up. Rates are user-supplied; no market data is embedded.
  • Structural opening, party wall matters and making-good are extension-specific costs absent from new build

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Frequently asked questions

Why does an extension cost more per square metre than a new house?
Because the site fights you. Materials come through an occupied building or round a narrow side passage, there is nowhere to store anything, the work stops for the people living there, and everything has to tie into a structure built to different standards decades ago. On top of that an extension carries two costs a new build does not: the structural opening into the existing house, and making good everywhere the new meets the old.
What is making good and why is it a separate line?
It is everything needed to turn two buildings into one house: levelling the floor between them, re-plastering the junction, extending heating and lighting circuits into the new space, and redecorating the rooms disturbed. It appears nowhere on the drawings, which is exactly why it appears as a surprise on the invoice. Giving it its own line is the point of this calculator.
Do I need a party wall surveyor?
In jurisdictions with party wall legislation, if you are building on or near a shared boundary or excavating within a defined distance of a neighbour's foundation, you must serve notice — and if the neighbour dissents, surveyors are appointed and you pay for them. It is not a cost you can decline, and it is worth establishing early rather than a fortnight before the dig.