What Is a Provisional Sum? (And When to Use One in a Building Quote)
A provisional sum is an allowance included in a building quote for work or materials that cannot be precisely defined at the time of quoting — typically because the scope is uncertain or specialist input is needed. It is an estimate, not a fixed price for that item, and it will be adjusted when the actual scope and cost become known during the project.
Used correctly, a provisional sum is a tool for transparency. It tells the client: "We know this item is coming, we don't yet know exactly what it will cost, and here's our best estimate." Used incorrectly — or without explanation — it becomes a source of disputes when the final account comes in higher than expected.
When to Include a Provisional Sum
A provisional sum is appropriate any time a specific element of the project cannot be accurately priced because the scope is not yet defined. The most common scenarios on UK domestic and commercial projects include:
- Asbestos removal — the full extent of asbestos-containing materials is usually unknown until a licensed surveyor has inspected or until the building is stripped out. A provisional sum allows the job to proceed while acknowledging that this line could go up significantly.
- Specialist steelwork — until the structural engineer produces their final design and beam schedule, the steelwork cost is unknown. A provisional sum based on an indicative spec is common at tender stage.
- Drainage repairs or alterations — when excavation reveals that existing drainage is not where the drawings show, or is in a worse condition than anticipated, the cost of repair or rerouting is genuinely unpredictable at tender stage.
- Made ground or obstructions — foundation costs on sites with made-up ground, contamination, or underground obstructions are extremely difficult to price accurately without trial holes.
- Landscaping or external works — if the client hasn't finalised the landscaping specification at the time of quoting, a provisional sum allows the main contract to be placed while the external works design develops.
Defined vs Undefined Provisional Sums (JCT)
Under JCT Standard Building Contract — the form most commonly used on UK commercial and larger domestic projects — there is a formal distinction between two types of provisional sum:
Defined provisional sum: Enough information exists for the contractor to include it in their programme of works and make preliminary arrangements. The contractor is required to allow for it in prelims and programme. If the actual instruction arrives late and disrupts the programme, the contractor may have a claim.
Undefined provisional sum: Insufficient information exists for any preliminary planning at tender stage. The contractor is not expected to include it in the programme. When the instruction is issued, additional time and cost are evaluated on their actual impact.
On domestic renovation projects, this formal JCT distinction is rarely applied in its technical sense — but understanding it helps builders structure their quotes more precisely and protect themselves commercially when time and cost implications arise from late instructions.
The Right Way to Use a Provisional Sum
After 32 years managing construction projects, I've seen provisional sums used well and badly. Here's the difference:
Used well: The quote says "Provisional sum for asbestos removal — £2,500. This is an estimate based on the visible materials present. A licensed asbestos survey will confirm the extent before this work is instructed. If the actual cost is lower, the saving will be passed to the client. If higher, we will agree the revised figure before proceeding." The client understands what they're agreeing to. There are no surprises.
Used badly: The provisional sum is set unrealistically low to make the quote look competitive. The client accepts on price. The actual cost comes in significantly higher. The builder raises a variation; the client refuses because the original "quote" was lower. A dispute follows that damages both the project and the relationship.
The principle is simple: if you're not sure, say so — and set your provisional sum at a realistic level. It is better to have a transparent allowance that comes in under than to guess low and fight over the difference.
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Try RenoCalc FreeFrequently Asked Questions
What is the difference between a defined and an undefined provisional sum?
Under JCT contracts, a defined provisional sum is one where enough information exists for the contractor to make preliminary arrangements — allowing time in the programme, arranging specialist access, etc. An undefined provisional sum is one where insufficient information exists for preliminary planning. The distinction matters commercially: on a defined provisional sum, a late instruction may not give grounds for a delay claim; on an undefined one, it may. In domestic projects, the distinction is rarely applied formally, but it's useful to understand on commercial work.
What are examples of provisional sums on a building project?
Common examples include: asbestos removal (unknown until survey or strip-out), specialist structural steelwork (depends on engineer's final design), drainage works (depends on what's found on excavation), groundworks on sites with potential obstructions, and any element where the specification is unresolved at tender stage. The allowance should be realistic, not artificially low — undersetting a provisional sum to win a job at the expense of the final account is poor practice that damages client relationships.
Can a contractor change a provisional sum?
Yes — and that is the whole point. A provisional sum is always adjusted once the actual scope and cost are confirmed. The contractor issues a variation removing the provisional sum and replacing it with the actual cost. If the actual cost is higher, the difference is added to the contract sum. If lower, the difference is credited. This must always be agreed in writing before the work proceeds — presenting a large variation on the final account is unacceptable practice.
What is the difference between a provisional sum and a prime cost sum?
A provisional sum covers work of uncertain scope — you know something will be needed but don't yet know what exactly. A prime cost (PC) sum covers a specific, known item where the supplier or product hasn't been chosen yet — scope is clear, price is not. Example: "Provisional sum for drainage — £3,500" (uncertain scope); "PC sum for kitchen units — £4,000" (known item, client yet to choose supplier).
How are provisional sums handled under JCT contracts?
Under a JCT Standard Building Contract, provisional sums are formally defined as either "defined" or "undefined". A defined provisional sum is where enough information exists for the contractor to plan preliminary work; an undefined one has insufficient information for any preliminary planning. When the instruction is issued, the provisional sum is omitted and replaced by an instruction valuation. For authoritative JCT contract information, see: JCT Ltd — Joint Contracts Tribunal.
Should provisional sums be realistic or conservative?
Always realistic. Setting a provisional sum artificially low to make a quote look competitive — and then raising it significantly during the project — is poor professional practice that damages client trust and leads to disputes. A well-set provisional sum based on realistic market data will sometimes come in under budget, which builds goodwill. One set too low will always create conflict.
