Self Build Budget
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How much contingency should I allow on a self build?

Short answer

Allow at least 10% of your build cost as contingency, and 15-20% if your project carries more risk. Penrith Building Society asks for evidence of a 15% contingency (Penrith Building Society), BuildStore recommends around 10-20% (BuildStore), Grand Designs magazine says 10-20% is important, and VELUX advises at least 10%. Keep it as a separate pot, use it only for genuine surprises, and release it as risks pass.

What the guides say

SourceContingency advised
Penrith Building Society (lender)Evidence of a 15% contingency
BuildStore (self build mortgage specialist)Around 10-20% of your build cost budget
Grand Designs magazine (updated 2022)10-20%
VELUXAt least 10%
Build It (2026 cost guide)Around 20% of the total budget, possibly more

The figures differ partly because they measure different things. A lender is checking your project is fundable; a magazine is trying to stop you getting caught out. Treat 10% as a floor, not a target.

Work the percentage out on the build cost, not the land. Build It’s cash flow guide frames it as about 10% of construction costs excluding land (Build It).

When to go higher

Move towards 15-20%, or beyond, if any of these apply:

  • Renovation or conversion. Existing buildings hide things. You often don’t know what is behind a wall or under a floor until it is opened up.
  • Unknown ground. Until you have a site investigation, foundations are a guess. Build It describes one project that met an unexpected £20,000 for deeper foundations (Build It, 2026).
  • First self build. You don’t yet know what you don’t know. Small items and forgotten costs add up.
  • Complex or bespoke design. Unusual shapes, large glazing, cantilevers and one-off details are harder to price and easier to get wrong.
  • Off-site systems with early deposits. Timber frame and system builds often need a large deposit before manufacture and full payment once the structure is up (Build It). BuildStore notes that timber construction costs more than brick and block in the early stages, because you pay for the whole system up front (BuildStore). The risk here is as much about timing as total cost.
  • Lots of provisional sums in your quotes. Every allowance that isn’t a firm price is a place where cost can move.

A simple, well-specified new build on surveyed ground with firm quotes can sit nearer 10%.

How to hold contingency

Keep it as its own line. Don’t spread it invisibly across every stage. If it is hidden, you can’t see how much is left, and it gets spent without anyone deciding to spend it.

Don’t spend it on upgrades. Contingency is for things you couldn’t have known about: bad ground, a price rise, a missed item, a delay. It is not for the nicer kitchen or the bigger rooflight. Build It is clear that it is for problems, not extras. If you want an upgrade, find the money elsewhere in the budget or agree to spend contingency on it as a deliberate decision, written down.

Decide together how it is used. If two of you are running the budget, agree a simple rule: any draw on contingency is logged with a reason and both of you know about it.

Keep it in cash, if you can. A contingency that only exists on paper doesn’t help when a supplier needs paying on Friday.

Releasing contingency as risk falls

Most of the risk in a self build sits early: ground, foundations, structure, getting the building watertight. Once you are out of the ground, a large part of the unknown has gone. Once the structure is up and the roof is on, more has gone.

So review contingency at each milestone. Ask what risks are left and whether the remaining pot still covers them. If you have a healthy amount left after the superstructure, you might free some of it, but be careful. Self builders often hit trouble towards the end, when finishes, kitchens, external works and all the small items arrive at once. Our page on why self builds go over budget covers that end-of-build squeeze. A sensible approach is to release only part of what is left and keep a solid buffer until completion.

Ranges and contingency work together

A ranged estimate and a contingency do different jobs. The range (a low and a high figure) covers uncertainty about what your design will cost while it is still on paper. Contingency covers things that go wrong once you are building.

At the start, plan on the high end of the range and add contingency on top. As quotes come in and you accept them, the range narrows to real numbers. Contingency then stays as your protection against the things no quote can cover. If you only use the low end of your range plus 10%, you are really running with no contingency at all.

How Self Build Budget helps

In Self Build Budget, contingency is tracked as its own budget, so you always see how much is left and what it has been used for. Because each line shows a projected cost based on what you have spent and accepted, you can see when a line is heading over budget before you have to raid contingency. Contingency is part of the Standard plan (£12 a month).

Questions people also ask

Is 10% contingency enough for a self build? For a simple new build with surveyed ground and firm quotes, it can be. For renovations, first-time builders or complex designs, 15-20% is safer.

Does my lender need to see contingency? Usually, yes. Penrith Building Society, for example, asks for evidence of a 15% contingency. Check your own lender’s requirements.

Should contingency include VAT? Calculate it on the same basis as the rest of your budget. If your budget includes VAT, so should your contingency.

What if we don’t use it? Then you finish under budget. Resist the temptation to spend it on upgrades halfway through; the end of the build is where money tends to run short.

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Sources