Whole Build & Extension Planner

Budget, timeline and build order for a UK extension or whole house, in one place. Pick your size, spec, region and site conditions to get three cost scenarios, a week-by-week schedule and a stage-payment plan you can take to builders.

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How it works

Base build cost per m²: extensions £1,850 (standard) to £3,200 (luxury); whole houses £1,650 to £4,200.
Regional adjustment: London ×1.25, South East ×1.12, Midlands ×1, North ×0.92, Scotland & Wales ×0.95.
Site extras: clay soil adds 10% to groundworks, rock adds 25% to excavation, poor access adds 12% to labour and plant. Trees and old consumer units are also priced in.
Kitchens and bathrooms are priced on top, then professional fees (12–13%) and VAT (20%, or 0% for new-build homes).
Time is split into six phases, from approvals to handover, each ending with an inspection checkpoint.

Limits

These are Calcurly's own UK 2026 planning estimates, not quotes. They don't cover demolition, basements, structural surprises, land costs or finance. Always get at least three itemised tenders.

Who it's for

Homeowners setting a budget before speaking to an architect, self-builders planning cash flow, and builders sanity-checking a tender.

UK examples

Illustrations calculated by this planner's own model with a 10% contingency. They are not real quotes or completed projects — get itemised tenders for your build.

Single-storey kitchen extension, Midlands

30 m², standard spec, good access, new kitchen.

Best case: £78,948
Realistic: £91,590
Worst case: £101,265
Timeline: 26 weeks

Two-storey extension, London

50 m², premium spec, clay soil, moderate access, kitchen + 1 bathroom.

Best case: £225,418
Realistic: £261,514
Worst case: £289,139
Timeline: 35 weeks

New 4-bed house, North of England

150 m², standard spec, two storeys, 2 bathrooms (new build, zero-rated VAT).

Best case: £260,440
Realistic: £305,087
Worst case: £342,292
Timeline: 63 weeks

Frequently asked questions

How accurate is the estimate?: It gives a planning range, not a quote. The realistic figure is a starting budget; real tenders can differ by 15% or more depending on design, specification and local demand.
Does it include VAT and professional fees?: Yes. Fees (architect, engineer, Building Control) are added at 12–13%, and VAT at 20% for extensions. New-build homes are zero-rated, so no VAT is added.
Why are there three scenarios?: Builds rarely go exactly to plan. Best case assumes no surprises, realistic adds your contingency, and worst case adds a further 15% for common overruns.
How should I pay my builder?: Pay in stages after each inspection, keep any deposit at 10% or less, and hold back 2.5–5% retention until snagging is complete.
Do I need planning permission?: Many single-storey rear extensions fall under permitted development, but larger or two-storey work usually needs planning. Always check with your local council.