Costs, extensions and planning
Building work is priced in ranges because it is delivered in ranges. Anyone giving you a single confident number for a job they have not seen is guessing — here is how to work out what yours should cost.
Compare scopes, not totals
Write your own schedule of work before you ask anyone to price it, then put every quote onto the same line-by-line list. The gaps appear immediately: one has excluded the kitchen, another has left out the making good, a third has not allowed for the drain diversion. A quote 30% below the others is almost always missing scope rather than being generous.
Budget for the things nobody quotes for
Flooring, blinds, lighting, garden reinstatement and decorating the rooms you disturbed will add £5,000 to £15,000 to a typical extension that no builder mentioned. Foundations that have to go deeper than expected. A build-over agreement for a drain in the wrong place. Ten percent contingency on a straightforward site, fifteen on anything pre-war.
The VAT relief almost nobody claims
If a property has stood empty for two years or more immediately before work starts, most renovation work qualifies for 5% VAT instead of 20%. On a £120,000 project that is around £15,000. The builder has to apply the rate at invoice stage, so it must be raised before they price the job — and you need evidence of the empty period, usually from the council’s empty property officer.
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