Contingency in construction is money you set aside for problems nobody could see before work started, such as rot in the timbers, asbestos or hidden structural defects. The usual guide is 10–20% of the build cost; with a fixed quote, thorough surveys and good drawings, 5–10% is often enough. Tolerance is the small, normal gap between the drawings and the real building, a few millimetres here and there, which a good builder deals with on site.
Understanding both makes a building project far less stressful. In this guide we explain how much contingency to allow, what it should and shouldn’t be spent on, and why small tolerances are part of every build.
Contingency in construction: how much should you allow?
A contingency is a reserve in your budget for genuine surprises. It isn’t part of the builder’s price; it’s your money, held back in case it’s needed.
| Your situation | Suggested contingency |
|---|---|
| Fixed, itemised quote, full drawings, surveys done, house in good condition | 5–10% |
| Typical loft conversion or extension | 10% |
| Older house, unknown condition, estimate rather than a quote, or major structural work | 15–20% |
The more you know before you start, the less you need. A detailed survey, proper loft conversion plans and a fixed quote all reduce the risk of surprises. Our guide to estimate vs quote explains why a fixed quote matters.
What contingency is for
Things that couldn’t reasonably be known until the work started, for example:
- Rot or woodworm found when the old ceilings and roof timbers are opened up
- Asbestos in an older house (any home built before 2000 may contain some), which must be tested and removed safely
- Bats found in the loft, which are protected and can delay or change the work
- Hidden structural problems, such as a chimney breast removed below without support, or weak lintels
- Tree roots or poor ground when digging foundations for an extension
- Old wiring or pipework that has to be replaced once it’s exposed
What contingency isn’t for
Contingency in construction should not be spent on upgrades and extras you decide to add along the way, such as a bigger rooflight, a better bathroom suite or an extra wardrobe. That’s scope creep, not a surprise. If you want those, budget for them separately, agree the price in writing, and keep the contingency for the unexpected.
Fixed quotes and provisional sums
Even with a fixed quote, some items may be priced as provisional sums because they can’t be known in advance, such as foundations or repairs to hidden timbers. Your contingency is what covers those if they come in higher. Ask your builder to list every provisional sum and assumption, so you know where the risks are.
Tolerance in construction: why the site never matches the drawings exactly
Tolerance is the allowable difference between what’s on the drawings and what’s built. Houses, especially older London houses, are rarely perfectly square, level or plumb, and products are made to their own sizes. So small adjustments on site are normal and expected.
Some everyday examples from our own projects:
- Windows: a window frame arrives a few millimetres smaller than the opening, so the opening is packed or trimmed to fit.
- Showers: the shower head is moved slightly to avoid a ceiling joist that isn’t quite where the drawings showed.
- Kitchens and fitted furniture: units are packed out to sit straight against a wall that isn’t.
- Drains and paving: a drainage channel is shifted to line up with the paving pattern.
- Loft rooms: the old roof isn’t exactly symmetrical, so wardrobes under the eaves or rooflight positions are adjusted by a few centimetres.
None of these is a mistake. A good builder spots them, makes the sensible adjustment and tells you about anything you’d notice. Going into a project expecting small tolerances, rather than millimetre perfection, saves a lot of stress, delay and cost.
When a tolerance becomes a defect
There’s a difference between a small, sensible adjustment and work that’s simply wrong. Walls that are visibly out of line, doors that don’t close, or a staircase that doesn’t meet the Building Regulations aren’t tolerances; they’re defects to fix. Our snagging guide explains how to check the finished work.
Example: budgeting for a loft conversion with contingency
Here’s how a sensible budget might look for a rear dormer loft conversion in London:
| Item | Amount |
|---|---|
| Fixed quote for the loft conversion (including fees) | £65,000 |
| Your extras, agreed in advance (upgraded bathroom, built-in wardrobes) | £5,000 |
| Contingency in construction at 10% of the build | £6,500 |
| Total budget | £76,500 |
If nothing unexpected turns up, you keep the £6,500. If the builder finds rot in the roof timbers, it’s covered without stress. Prices for each type of loft are in our guide to the different types of loft conversion.
If you’re buying a house to convert, factor the contingency in before you make an offer; our guide to buying a house for a loft conversion shows what to check. And choose a builder who explains the risks openly; our guide on how to choose a builder covers what to ask.
FAQs
What is contingency in construction?
A reserve in your budget for problems that couldn’t be known before work started, such as rot, asbestos, bats or hidden structural defects. It’s money you hold back, not part of the builder’s price.
How much contingency should I allow for a loft conversion?
Around 10% of the build cost for a typical loft conversion. With a fixed quote, full drawings and a house in good condition, 5–10% is usually enough; for an older house in unknown condition, allow 15–20%.
Is contingency included in a builder’s quote?
Usually not. A quote covers the work described. Some quotes include provisional sums for unknowns, but the contingency is normally your own reserve on top.
What’s the difference between contingency and tolerance in construction?
Contingency is money for unexpected problems. Tolerance is the small, normal difference between the drawings and the real building, a few millimetres here and there, which a good builder adjusts for on site.
Should I use contingency for upgrades?
No. Contingency in construction is for genuine surprises. Budget separately for any upgrades or extras, and agree their price in writing before the work is done.
What happens to the contingency if I don’t need it?
You keep it. That’s the point: it’s a cushion, not a target.
Plan your loft with fewer surprises
We survey every loft before we price it, list our assumptions in writing and give you one fixed quote, so your contingency stays where it belongs: in your pocket.
