How to budget a building project

Setting a realistic budget for a building project means going beyond the headline quote and building in room for the things that predictably go wrong on real jobs, even well-planned ones, rather than assuming the first number a builder gives will be the final one.

Contingency is the single most important line in any building budget, and how much to set aside depends heavily on what's actually being built. A sensible contingency for a standard new extension on a straightforward site is commonly cited around 10% on top of the quote. Renovating an older property, where walls, floors and services are being opened up and their condition is genuinely unknown until work starts, commonly warrants 15-20%. Listed buildings or significant structural alteration push that further still, commonly 20% or more, reflecting both the technical uncertainty and the extra approvals often involved.

That contingency isn't a padding figure to quietly not spend; survey data commonly cited shows around 38% of UK renovators go over budget, by an average of roughly 20%, with only a small minority coming in under budget. The overrun is rarely driven mainly by material prices moving; it's more commonly driven by the original quote under-counting labour and professional fees, and by the condition exposed once plaster or floorboards actually come up, damp, rot or old wiring being common examples, plus any compliance upgrade Building Control ends up requiring once the work becomes formally notifiable.

Understanding what actually drives cost helps explain why two quotes for what looks like the same job can differ so much. Specification level, whether that's a standard fitted kitchen or a bespoke one, and a mid-range bathroom versus a high-end one, moves the total substantially even when the structural work is identical. Site access matters more than most people expect: a project where materials can be delivered straight to the working area costs less to build than one requiring everything carried through a house or hoisted over a boundary. Structural complexity, an open-plan knock-through with substantial steelwork versus a simple like-for-like extension, adds cost regardless of finish level. And the underlying condition of the existing building, whether foundations, drainage or timber are sound or need remedial work before the new build can even start, is often the single biggest unknown at quote stage.

A sensible payment schedule protects both sides of the arrangement and should be agreed as part of the written contract before work starts, not negotiated partway through. Payments tied to specific completed milestones, foundations poured, structure watertight, first fix complete, and so on, rather than to calendar dates, keep what's been paid roughly in line with what's actually been delivered at any point in the project. A reasonable deposit is commonly 5-10% of the contract price; a schedule that front-loads payment well ahead of the work being done shifts risk heavily onto the homeowner if a builder becomes unavailable or the relationship breaks down partway through.

Snagging, the list of minor defects and unfinished details spotted once the main work is done, should be built into the contract from the outset rather than treated as an afterthought. The contract should commit the builder to a final walkthrough with the homeowner and to returning to fix snagging items within a defined period, and for a larger project, an independent snagging survey, commonly £300-£800, provides a more objective and thorough list than a homeowner spotting issues alone, particularly for anything structural or related to weatherproofing that isn't obvious on a casual look.

A quote sitting 30-40% below every other one received is worth treating as a warning sign rather than good fortune. It commonly reflects one of a handful of underlying issues: cash-in-hand working with no proper insurance behind it, cheaper materials substituted without being disclosed upfront, a scope that's been significantly underestimated and will resurface as costly "extras" once work is underway, or unlicensed labour being used on notifiable work such as electrics. Comparing quotes on headline price alone is genuinely risky, since one figure can quietly exclude scaffolding, skip hire, building control fees or making good to existing surfaces, while a higher-looking quote from another builder includes all of it; checking exactly what's included, and what isn't, before comparing the numbers is worth the extra half hour it takes.

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