SettingsSettings for this calculationUS
New internal floor area, all storeys.
Internal area of the NEW space, added up across every storey — a two-storey extension of 20 m² per floor is 40 m² here. Measure inside the finished walls. Do not include area you are reconfiguring inside the existing house: that work is real but it is not extension rate work, and the two are priced differently because one of them has a roof already.
Your rate for the extension shell and fit-out.
Extension rates run above new-build rates for the same finish. The site is constrained, materials are carried through an occupied house, and the work stops and starts around the people living there. This field is read PER SQUARE METRE however the unit switch is set: the floor area above follows the switch and this rate does not, because there is no dimension for money per unit area, so the number is multiplied exactly as typed. A rate off a US price book is quoted per foot squared — the unit the floor area field above switches to — and there are 10.764 of those in a square metre, so multiply your rate by 10.764 before it goes in here. The 2,600 per m² this field offers as an example is 241.55 in the other denominator, and 241.55 sits well inside this field's 0 to 100,000 range: it is taken in silence, with no warning and no clamp, and the shell line comes back at 9.3% of the cost you meant. The fixed sums below dilute that, so on the example figures the total still lands near a quarter of the right answer rather than a tenth — wrong, and not obviously wrong.
Beam, padstones, temporary support, engineer.
Removing part of the existing rear wall needs a designed beam, bearings sized for the load, temporary propping while it goes in, and structural calculations for building control. Wide openings and any opening under a chimney escalate quickly.
Where new meets old.
Levelling floors between the two, re-plastering the junction, extending heating and electrical circuits, and redecorating the rooms affected. It is invisible on a plan and it is always there in the invoice.
Architect, engineer, planning and building control.
Include the planning application fee and building control charges, and a party wall surveyor where the work is near or on a shared boundary — that appointment is not optional if a neighbour dissents.
Fifteen percent minimum on an extension.
You are building against an existing structure whose foundations, drainage and construction you do not fully know until you dig. Underpinning next to a shallow existing footing, or a drain in exactly the wrong place, are both common and both expensive.
As it applies in your jurisdiction.
Extensions are often taxed differently from new build. Check your own position.
Total extension cost
Needs your Build rate (per m²)
This page does not assume a price. Enter yours and the answer appears here.
They open the calculator with your figures already in it
Home Extension Cost Calculator — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- 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
Inputs used
- Extension floor area
- 320 sq ft
- Build rate (per m²)
- Structural opening
- 0
- Making good the existing house
- 0
- Design, planning and approvals
- 0
- Contingency (%)
- 15
- Sales tax / VAT (%)
- 0
What this calculation does not cover
- Excludes underpinning, which is a specialist operation priced by the linear length and is only known to be needed once trial holes are dug.
- Excludes diverting a public sewer, which requires the water authority's agreement and is priced by them.
- Excludes rehousing or storage while the work runs, which on a rear extension into the only kitchen is a real cost.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-05 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations2
- 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
Cite this page
Your workspace
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