Free Estimation Tool

Extension Cost Calculator

Get an instant rough estimate for your home extension. Adjust the size, add windows, doors and skylights — your live cost updates as you go.

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⚠️
These are estimated guide prices only. Final costs depend on site conditions, specification, materials and finishes. Structural steelwork in particular is highly variable and must be assessed during a site visit — the figure shown is indicative only. A formal quotation will be provided after survey.
Extension Type
Extension Size @ £1,800 / sqm
m²
Windows @ £850 / m²
0
Bifold / Sliding Doors @ £2,500 / m
0.0m
Doors @ £350 / m²
0
Skylights / Roof Lights @ £1,200 each
0
Structural Steelwork @ £2,000 / m
0.0m
⚠️ Structural steel is highly variable. This figure is a rough guide only — actual cost must be confirmed on site.
Strip Foundations concrete @ £120 / m³
Perimeter estimated from floor area (3-wall rule). Confirmed on site survey.
Floor Construction hardcore · sand · insulation · cavity batts
Flat Roof Construction OSB deck + covering
Estimated Cost
£54,000
Range: £48,600 – £59,400
Optimistic −10% £48,600
Higher End +10% £59,400
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This is a rough guide only ±10%. Final costs depend on specification, site conditions, materials and finishes. A formal quote will be provided after survey.

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