Free UK planning calculator
Loft & Extension Calculator
Build a clearer UK planning budget from project type, floor area, location, finish level, site access and major fit-out choices.
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Itemised estimate
Based on your choices
1. Project and location
2. Specification and scope
Estimated cost breakdown
How this estimate is calculated
Project type sets the starting construction rate. The rooflight setting uses £750–£1,000 per m², derived from the published £22,500–£30,000 30 m² guide example. This is an early benchmark, not a guaranteed like-for-like quotation. Other project types retain their separate planning rates.
Location and access affect build costs. London and the South East adds 20% to modelled construction and fit-out; restricted access increases labour and preliminaries.
Finish level changes internal fit-out, not the whole structure. En-suites and kitchens are shown separately so they do not disappear into a broad rate.
Professional costs and contingency stay visible. They can be included or excluded rather than hidden inside the headline total. VAT is not separately calculated; check its treatment in each quote.
Sources and methodology: read how our planning estimates are built and the sources behind them.
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