Renovation contingency UK: how much to keep aside
The standard rules by project type and property age, the surprises contingency exists for, and the discipline that keeps it ringfenced.
Contingency is the most-skipped line in a UK renovation budget. It's also the one that determines whether your project finishes on plan or in panic. The good news: there's a working answer for how much to set aside, and it's not as much as you'd fear once you understand what it actually covers.
This guide walks through the standard contingency rules by project type and property age, the typical surprises contingency exists for, the discipline that keeps it ringfenced, and what to do if you've spent it before the work is done.
What contingency is, and why it's not optional
Contingency is a ringfenced part of your renovation budget set aside for things you can't predict before the work starts. Hidden defects. Materials shortages. Variation orders that emerge once walls are open.
It's not the budget for nice-to-haves. The mid-project upgrade to a better tap, the lighting plan you decided on after seeing the room, the planter you saw on Pinterest: those are scope creep. Real contingency exists to absorb genuine unknowns.
The Federation of Master Builders, HomeOwners Alliance and RICS all recommend setting contingency from the start as a separate line in your budget. The benchmark figures vary by project type and property age (the next section walks through the rules), but the principle is consistent: ringfence it, don't dip casually, and treat what's left at the end as a return to savings, not a windfall.
How much contingency to keep, by project type and property age
The rule is two-dimensional: project type drives one number, property age drives another, and you take the higher of the two.
By project type:
General renovation (kitchen, bathroom, redecoration, minor works): 10–15% of total budget.
Extensions (single-storey, double-storey, side return, wraparound): 15–20% of total budget. Extensions have more unknown variables (foundations, ground conditions, party walls) and a longer construction window. For the full breakdown of extension-specific cost variables, see our complete extension costs guide→.
By property age:
Post-1980 properties: stick to the project-type rule above.
Pre-1960 properties (Victorian, Edwardian, 1930s, 1950s): 20%+ regardless of project type. Older houses surprise their owners more often.
Listed buildings or properties in conservation areas: 25%+. Specialist materials, conservation requirements, and slower trade availability all push contingency higher.
Take the higher of the two. A bathroom refit on a Victorian terrace runs at 20%+ (property age wins). An extension on a 1990s semi runs at 15–20% (project type wins). An extension on a Victorian terrace runs at 20%+ (whichever is higher).
The surprises contingency exists for
Most renovations turn up at least one surprise. Older houses turn up more. Here's what we found in our own Victorian terrace renovation, plus what we typically hear from other homeowners going through similar work.
What we found:
Damp from two different sources. Rising damp at the front-room bay window, which we tackled when we first moved in, and a separate problem at the rear of the house twelve months later, where 1980s retrofitted pipework failed and decanted water down the inside of the brickwork. The front-room job was the bigger of the two: damp-proofing specialists had to proof the entire wall contact surface around the window, down both sides and across the top. The rear pipe surfaced a full year after we'd moved in, well outside any renovation budget we'd set. It was an unwelcome reminder that not every surprise turns up neatly during your project window.
A consumer unit removed in a hazmat suit. The original electrical system needed to be fully pulled out and the main unit replaced. The electrician handling the removal arrived in full protective equipment. We can only assume because of what he expected to find inside. Pre-1980s wiring rarely meets current standards on its own; sometimes the bigger surprise is in the components themselves.
Builder's waste under the garden. When we started digging beds and laying paving, half the garden turned out to be glass, bricks and broken concrete buried just below the surface, left over from earlier work on the house (the same 1980s installers, possibly). Worth a contingency line on landscaping budgets too, especially if earlier renovation work has happened on the site.
What we typically see beyond that:
Rotten timber in joists, floorboards or roof timbers, especially where water has been getting in for years. Structural issues uncovered during strip-out: sagging joists, undersized lintels, settlement cracks that turn out to be live rather than historic. Asbestos in artex ceilings, textured coatings and pipe lagging; removal is licensed work. Drainage and soil-stack problems in older clay runs.
The cost of an unplanned partial or full rewire runs £3,500–£7,000+ for a typical 3-bed, to give a sense of the magnitudes involved.
The discipline: how to manage contingency
Setting the number is half the battle. Holding it is the other half.
Ringfence it as a separate budget line. Not part of materials, not part of labour. A separate row with a running balance you check before any unplanned spend.
Don't spend it on upgrades. A nicer tap, a different worktop, an extra coat of paint: those are scope creep. They come out of the relevant category line, not contingency. Once contingency starts paying for upgrades, it isn't there for the surprise.
Treat unspent contingency as savings, not a windfall. If you finish the project with 4% contingency left, that goes back to your account or into the snagging budget for fixes that emerge after handover. It does not become the holiday fund.
Check the burn rate at the half-way mark. If you're 60% through the project and haven't touched contingency, revisit the remaining figure in the context of the work still to come. It doesn't mean spend it. It means you can probably plan the second half with confidence.
When you've spent the contingency, what then
Sometimes contingency runs out before the work is done. Pre-1960 properties are particularly prone, and so are projects where the initial scope was loosely defined.
Pause and reassess scope honestly. What's left to do? What's optional versus essential? Could anything be deferred to a later phase rather than crammed into this budget?
Cut nice-to-haves before essentials. The bespoke wardrobe can wait. The structural beam over the new opening cannot.
Know which lines are fixed and which are flexible. Structural and regulatory work is fixed. Finishes are usually flexible. Mid-range tiles cost a fraction of premium ones for the same finished look.
Treat it as a budget conversation, not a crisis. Talk to your contractor about phasing, cheaper alternatives within the existing scope, and which decisions are reversible later.
For broader cost context across the project, see our complete guide to UK renovation costs→. If you're earlier in the process and want to anchor the broader plan, see our complete first renovation guide→.
Frequently asked questions
Honely's cost benchmarks have contingency built in
House-specific and project-specific contingency baked into our cost benchmarks, so you know the total amount you should budget from the start.
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