Every construction budget has one. A single line, sitting quietly in the bill of quantities, that looks harmless enough. “Provisional Sum: €15,000.” It’s easy to skim past it, nod, and move on to the next item. That’s exactly how it catches people out.

Provisional sums are one of the most misunderstood parts of a construction budget, and one of the most common reasons a project’s final cost ends up nowhere near the figure everyone agreed at the start. If you’re planning a build, whether it’s a self-build, a renovation, or a commercial fit-out, understanding how provisional sums actually work could save you a very uncomfortable conversation further down the line.

What a Provisional Sum Actually Is

A provisional sum is an estimated allowance included in a contract for work that can’t be fully defined or priced at tender stage. It’s not a fixed price. It’s a placeholder, put in by the quantity surveyor as a best estimate for something that will be finalised later, once more information is available.

You’ll typically see provisional sums used for things like groundworks on a site where ground conditions aren’t fully known, specialist items that haven’t been selected yet, statutory connection fees that depend on the utility provider, or work that depends on a decision the client hasn’t made.

The key word is provisional. It’s a placeholder built into the contract sum, not a guarantee of what the work will actually cost.

Why This Catches So Many People Out

Here’s where the trouble starts. Clients often read a provisional sum the same way they read every other line item on the budget, as a fixed number. It’s in the contract, it’s in the total, so surely it’s covered.

It isn’t. When the actual scope of that work is finalised, whether it’s the ground conditions revealing themselves once digging starts, or a specification being confirmed, the provisional sum gets adjusted to reflect the real cost. If the real cost is higher than the allowance, and it very often is, that difference gets added to the final account. The budget you thought was locked in moves, and it moves in one direction far more often than the other.

This isn’t a builder trying to catch you out. It’s simply how provisional sums are designed to work. The problem is that far too many people don’t understand that going in, and the surprise lands at the worst possible time, when the work is already underway and there’s no room left to adjust the rest of the budget.

Where Provisional Sums Cause the Most Damage

Groundworks is the classic example. Nobody knows exactly what’s beneath a site until the digger starts moving earth. If rock turns up where soft ground was expected, or if drainage needs are more extensive than anticipated, that provisional allowance can be exceeded quickly, and by a significant margin.

Statutory and utility connections are another common flashpoint. The provisional sum put in at tender stage is often based on a general assumption, not a confirmed quote from the utility provider, and the real figure can land considerably higher once it’s confirmed.

Specialist fit-out items, kitchens, sanitaryware, ironmongery, are also frequently provisional, simply because the client hasn’t made a final selection yet. The allowance in the budget reflects a mid-range assumption, and if the eventual choice sits above that level, the difference lands on the final account.

How to Protect Your Budget

The single most important thing you can do is treat every provisional sum in your budget as a risk, not a certainty. Ask what it’s based on, how confident the allowance is, and what would happen to your overall budget if it came in higher.

Build contingency around provisional items specifically, not just as a general buffer for the whole project. A provisional sum for groundworks on a site with unknown conditions deserves more caution than one for a fitting that’s almost certainly going to land close to the allowance.

Push for provisional sums to be firmed up and converted into fixed prices as early as possible. The longer an item stays provisional, the longer it stays a risk sitting inside your budget rather than a number you can actually rely on.

And get a proper cost plan built around your actual project, not a generic template, so every provisional allowance reflects your site, your spec, and realistic current market conditions rather than a rough placeholder carried over from another job.

Getting It Right From the Start

Provisional sums exist for good reason. Nobody can price what hasn’t been decided yet, and pretending otherwise would just push the risk somewhere less visible. But a budget full of unmanaged provisional sums isn’t really a budget at all, it’s a guess with a number attached.

A properly prepared cost plan identifies exactly where the provisional risk sits in your project, sizes it realistically, and keeps it visible right through to final account, so nothing arrives as a surprise. That’s the kind of cost control we build into every project at Swiftcore, from initial estimating right through post-contract cost management. If you’d like a clear view of where the provisional risk sits in your own budget, get in touch with Swiftcore and we’ll help you plan for it properly, not just hope it doesn’t happen.