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Set a contingency

A contingency is money set aside for things that will go wrong but cannot be predicted. Large-project guidance says the amount should come from a risk review, and self-build guidance gives typical ranges.

How

1. List the main risks: unknown ground conditions, price rises, weather delays, permit conditions, design changes, labour shortages.
2. UK self-build guidance examples: the National Self Build and Renovation Centre suggests roughly 10 to 15 percent of the build budget; the Self Build Portal suggests 15 to 20 percent and about 20 percent for sloping sites or uncertain ground.
3. Choose a percentage at the higher end if your site, design or methods are unusual, if you are doing much of the work yourself for the first time, or if prices are volatile.
4. Hold the contingency as a separate pot. Do not spend it on upgrades.
5. Record every draw on the contingency with the reason, so you learn where your estimate was weak.
6. Use the budget calculator in this module to add the contingency to your system budgets.

Sources

1. Cost Estimating and Assessment Guide: cost estimating challenges. US Government Accountability Office, accessed 2026-11
2. Cost Estimating and Assessment Guide: allocating, phasing and converting a risk adjusted cost estimate. US Government Accountability Office, accessed 2026-11
3. The costs of a self build project. National Self Build and Renovation Centre (NSBRC), accessed 2026-11
4. Budgeting for your build project. Self Build Portal (NaCSBA), accessed 2026-11
5. GAO-20-195G Cost Estimating and Assessment Guide (PDF). US Government Accountability Office (hosted by US Department of Energy), accessed 2026-11

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