UK guidance says 10 to 20% on top of your contract sum. Which end applies to your project depends on one thing: how much of the existing building the work opens up. Here is the range per project type, with the reasoning.
20% contingency
Structural work on the existing building carries the most unknowns; guidance puts these projects at the top of the contingency range or above.
Loft conversion · Rear extension · Side return extension · Full house renovation
15 to 20% contingency
This work opens up the existing fabric, where damp, rotten timber or outdated wiring only show once things are apart.
Garage conversion · Rewiring (3-bed reference) · Damp proofing · Wet room · Bathroom renovation
10 to 15% contingency
Standard guidance for renovation work, with room to sit higher in an older property.
New boiler / central heating · Kitchen renovation
10% contingency
Surface-level work with little opening up; the lower end of the guidance range is usually enough.
Windows & doors · Driveway & patio · Painting & decorating
Source: Federation of Master Builders budgeting guidance (10 to 20% contingency; the top end for structural work and older properties), checked August 2026. The same advice shows under your result in the cost calculator.
Because the risk is not evenly spread. Painting a room rarely surprises anyone; opening up floors and walls in an older house does. UK builder guidance therefore gives a range, 10 to 20%, and puts structural work and older properties at the top of it or above.
No. Contingency is money you ringfence yourself on top of the contract sum, for things nobody priced: hidden damp, rotten timber, outdated wiring, ground surprises. If the builder includes a provisional sum for a specific risk, that is separate from your own buffer.
Track it. Log every extra with its amount and whether it was agreed in writing, and watch your total spend against the contract sum. The free extras log and budget tracker on this site do exactly that, on your own device.
Yes. The guidance is explicit: in older homes hidden issues such as damp, rotting timber or outdated wiring are more likely to surface once work begins, so allow the top of the range, and for listed buildings or structural alterations 20% or more.