How Much Contingency Should You Hold for Building Work?
The quote is £95,000. Your budget is £95,000. That is how people end up in trouble halfway through a build. Contingency is the money you hold back for the unknowns the quote cannot see yet. It is not a tip for the builder, and it is not the same as a provisional sum.
Quick answer
- Hold about 10% of the build cost as contingency on a typical extension or renovation.
- Hold about 15% if the property is pre-war, or the job involves a lot of opening-up, or the quote is thin on detail.
- Contingency sits with you, not as a loose line the builder can spend without agreement.
- Provisional sums are separate: they are allowances inside the quote. Contingency covers what nobody priced.
- The less detailed the quote, the bigger the contingency you need.
If the quote is vague, send it over before you set the budget. For £59 I'll tell you where the gaps are. Review my builder's quote, £59
Contingency vs provisional sum vs preliminaries
| Term | Whose money? | What it is for |
|---|---|---|
| Preliminaries | In the quote | Scaffold, skips, welfare, supervision (explained here) |
| Provisional / PC sums | In the quote, but not fixed | Known unknowns with an allowance (explained here) |
| Contingency | Your reserve | Unknown unknowns and decisions you have not made yet |
Do not let a builder "include 10% contingency" as a soft pot they can draw on without a written variation. If they want a risk allowance inside their price, it should be transparent. Your contingency stays under your control.
How much contingency for an extension?
For a straightforward single-storey extension on a modern house, with proper drawings and a detailed quote, 10% of the build cost is the figure I tell homeowners to hold.
Move toward 15% when:
- The house is pre-war (Victorian, Edwardian, older terraces): hidden rot, non-standard construction, shallow foundations, shared drains.
- You are renovating while living on site and may change your mind about finishes.
- Ground conditions are uncertain and foundations are only loosely defined.
- The quote is a short lump sum with lots of exclusions or provisional sums.
These percentages match what we use elsewhere on this site: 10% of build cost, 15% for pre-war.
How much for a renovation or refurbishment?
Whole-house refurbs often deserve the higher end, especially with services (electrics, plumbing, heating) that only reveal their state once walls are open. If you are also applying for the empty-home VAT reduced rate, keep contingency separate from tax planning: VAT relief does not remove building risk.
What contingency is for (and not for)
For:
- Discoveries behind finishes (rot, wiring that fails inspection, unexpected steel)
- Design tweaks you choose mid-job
- Price movement on long-lead items you had not locked
- Making good that only becomes obvious once work starts
Not for:
- Paying a huge deposit (keep deposits to 5-10%)
- Covering a builder who under-priced a fixed scope
- Absorbing variations you never agreed
The less detailed the quote, the more you need
A clear schedule of works, drawings and a line-by-line quote shrinks risk. A two-page PDF with a single number expands it. Before you set contingency:
- Run the builder quote checker on each quote.
- Compare them with the comparison tool.
- Read why builder quotes differ.
Thin quote, tight budget? That is the dangerous combination. For £59 I'll read up to three quotes for one job and tell you how much uncertainty is still sitting in the paperwork. Review my builder's quote, £59
How to hold and release contingency
- Keep it in your account, not prepaid to the builder.
- Release it only against written variations you have agreed, or genuine provisional-sum overruns with evidence.
- At the end, leftover contingency is yours. Celebrate by fitting the nicer taps, or by putting it back in savings.
Worked example
Build cost on the winning quote: £100,000 (VAT-inclusive, after you compared like-for-like).
- Modern house, detailed drawings: hold £10,000.
- Pre-war terrace, some provisional foundations: hold £15,000.
- Budget you need available: £110,000 or £115,000, plus professional fees if they sit outside the build quote.
The short version
Plan on 10%, or 15% for pre-war and high-uncertainty jobs. Hold it yourself. Do not confuse it with prelims or provisional sums. The clearer the quote, the less contingency you need; the vaguer the quote, the more.
Want help sizing the risk in your quotes?
I've been a builder for nearly 30 years. For £59, send me up to three quotes for one job. I'll show you where the gaps are and how much uncertainty still sits outside the price. Written review and a call within 24 hours.
Review my builder's quote, £59 · See an example review · Free quote comparison
Frequently asked questions
How much contingency should I hold for an extension?
About 10% of the build cost on a typical well-defined extension. Hold about 15% for pre-war properties or jobs with a lot of opening-up or a thin quote.
How much contingency for a renovation?
Often toward 15%, especially where services and hidden fabric are uncertain. The vaguer the quote, the more you should hold.
Is contingency the same as a provisional sum?
No. Provisional sums are allowances inside the quote for defined unknowns. Contingency is your separate reserve for things nobody priced.
Should the builder hold the contingency?
No. Keep it in your account and release it only against written variations or evidenced provisional-sum overruns you have agreed.
What if my quote already has 10% contingency in it?
Ask what it covers and how it is authorised. A soft pot the builder can spend without agreement is not the same as your own contingency.
Does a detailed quote mean I can hold less contingency?
Usually yes. Clear drawings, a schedule of works and few provisional sums reduce uncertainty. A short lump sum means you should hold more.
Work it out yourself
Free tools built on real 2026 builder rates. No sign-up, no email.