If you’ve looked closely at a Bill of Quantities — including the sample table in our BOQ template guide — you may have noticed a line for “contingency” near the bottom, and wondered how it relates to another term you’ll often see in construction contracts: the provisional sum. The two are related but not the same thing, and understanding provisional sums properly can save you from a nasty surprise at final account. This guide explains what a provisional sum actually is, why it exists, and how to keep it from being misused on your project.
What Is a Provisional Sum?
A provisional sum is an estimated allowance included in a Bill of Quantities or construction contract for an item of work that cannot be accurately described, measured, or priced at the time the contract is signed — usually because the design isn’t yet finalized, the exact extent of the work is unknown, or conditions on site haven’t been fully investigated. Rather than leaving the item out entirely (which would make the contract sum incomplete) or guessing at a fixed price (which could badly under- or over-state the true cost), the parties agree to carry a reasonable estimated figure that gets adjusted to the actual cost once the work is properly defined and executed.
In practice, a provisional sum is a placeholder, not a final price. When the work it covers is eventually measured or priced accurately, the original provisional figure is removed from the contract sum and replaced with the real cost — a process handled during interim valuations and finalized in the project’s final account.
Why Provisional Sums Are Used
- Unknown ground conditions. Before excavation, you often can’t be certain exactly what the soil will require — the true extent of piling, extra excavation, or soil stabilization may only become clear once work starts.
- Design not yet finalized. Some elements — a feature staircase, a specialist finish, landscaping design — may still be under development when the contract is signed and tendered.
- Work dependent on external authorities. Connection fees for electricity, water, or drainage from a statutory authority are often unknown until the authority itself confirms them.
- Specialist scope not yet defined. Items like a security or fire system that will be designed by a specialist subcontractor later in the project are commonly carried as provisional sums at tender stage.
Suggested image: a soil investigation or ground condition survey being carried out at a construction site.
Suggested alt text: “Ground investigation work often covered by a provisional sum in construction contracts”
Defined vs Undefined Provisional Sums
Quantity surveying practice generally splits provisional sums into two categories, and the distinction matters for how a contractor is expected to price the rest of their tender:
Defined Provisional Sums
Used where the nature and scope of the work is broadly known, even though the precise quantity or final design detail isn’t. Because the contractor understands enough about the work to plan around it, they’re expected to include an allowance for programming time, site facilities, and general overheads related to that item within their main tender price — only the specific cost of the work itself is provisional.
Undefined Provisional Sums
Used where almost nothing is known yet about the nature, extent, or timing of the work. Since the contractor genuinely cannot plan around something this undefined, they aren’t expected to make any allowance for it in their programming or overheads — and if the item causes disruption or extra preliminaries once it’s defined, that becomes a legitimate additional claim.
Getting this classification right at tender stage avoids a common and expensive dispute later: a contractor arguing that an “undefined” item disrupted their programme and entitles them to extra time and overhead cost, when the owner believed it had already been priced in.
Provisional Sum vs Prime Cost (PC) Sum vs Contingency Sum
These three terms are often confused with one another, but each serves a distinct purpose:
| Term | What It Covers | When It’s Used |
|---|---|---|
| Provisional Sum | Work whose scope isn’t fully defined at tender stage | Ground conditions, undesigned elements, authority connection fees |
| Prime Cost (PC) Sum | A specific item or specialist subcontract where the supplier/installer may not yet be selected, but the item itself is defined | Named specialist works (e.g. lift installation, specific fittings) to be nominated later |
| Contingency Sum | A general allowance for unforeseen circumstances across the whole project, not tied to any specific item | Covers genuinely unexpected issues that arise during construction, typically 5–10% of the contract sum |
The key difference: a provisional sum and a PC sum both relate to identifiable — if not yet fully defined — items of work, while a contingency sum is a general risk allowance not linked to any particular item on the bill.
Suggested image: a Bill of Quantities document with a provisional sum line item highlighted or circled.
Suggested alt text: “Provisional sum line item highlighted in a construction Bill of Quantities”
Where Provisional Sums Typically Appear in a Nigerian Residential BOQ
- Soil investigation and geotechnical testing, where the full scope of foundation work depends on the results
- Site clearance where the extent of vegetation, debris, or existing structures isn’t fully known until work begins
- Electricity, water, and drainage connection fees payable to the relevant statutory authority
- Specialist installations (security systems, solar/inverter systems, borehole drilling) where design and specification are still being finalized
- External works and landscaping, where the final design is often deferred until later in the project
These are exactly the kind of items that, if forced into a fixed lump-sum price too early, tend to produce either an inflated contractor quote (padded to cover the contractor’s uncertainty) or a dispute later when the real scope turns out to be larger or smaller than assumed.
How Provisional Sums Are Adjusted at Final Account
Once the actual work covered by a provisional sum is carried out, it’s measured (or otherwise valued, for a lump-sum specialist quote) and priced at its real cost. This actual figure replaces the original provisional allowance in the final account — meaning the project’s final cost could end up higher or lower than the contract sum originally suggested, purely because of how the provisional items were eventually resolved. This is a normal and expected part of how a provisional sum works, not a sign that something has gone wrong with the project.
Common Problems and How to Avoid Them
- Overusing provisional sums. If too much of the contract value sits in provisional sums, you effectively lose price certainty on your project — closer to a cost-plus arrangement than a fixed-price contract. Push your QS or contractor to define as much as realistically possible before signing.
- Vague descriptions. A provisional sum with a one-line description (“electrical works – provisional”) gives little basis for checking whether the eventual actual cost is reasonable. Insist on a clear description of what the sum is meant to cover.
- Disputes over defined vs undefined status. Get this classification agreed and documented at tender stage, not argued about once a delay claim comes in.
- Assuming the provisional figure is the final price. Budget with the understanding that a provisional sum could increase once the real scope is known — don’t treat it as a hard ceiling.
Tips for Building Owners
- Ask your quantity surveyor to flag every provisional sum in the BOQ and briefly explain why each one couldn’t be firmly priced at tender stage.
- Where possible, push for design decisions to be finalized before tender so fewer items need to be carried as provisional sums in the first place.
- Track provisional sum expenditure separately during construction, so you always know how your actual spend compares to the original allowances.
- Review provisional sum adjustments carefully at each valuation, rather than waiting until final account to check them all at once.
Suggested image: a client and quantity surveyor reviewing an interim valuation statement together.
Suggested alt text: “Reviewing provisional sum expenditure during a construction valuation in Nigeria”
A Worked Example
Suppose a residential BOQ carries a provisional sum of ₦800,000 for borehole drilling and water system installation, because the exact depth needed to reach a reliable water table isn’t known until drilling actually starts. The contract sum is signed with that ₦800,000 figure included as an estimate. Once drilling begins, it turns out the borehole needs to go significantly deeper than assumed, and the actual measured cost of the completed work — drilling, casing, pump, and overhead tank connection — comes to ₦1,150,000.
At the next valuation, the quantity surveyor removes the original ₦800,000 provisional allowance from the contract sum and replaces it with the actual ₦1,150,000 cost, adjusted for the agreed contractor profit and overhead margin where applicable. The net effect is a ₦350,000 increase to the project’s overall cost — not because of a contractor error or a hidden charge, but because the provisional sum did exactly what it was designed to do: hold a placeholder until the real scope of an uncertain item became clear. Had the borehole instead needed less drilling than expected, the adjustment would have worked the other way, reducing the contract sum by the difference.
This example is also why it matters to ask, at tender stage, roughly what assumptions sit behind a provisional figure — in this case, an assumed drilling depth — so you have a realistic sense of how much the number could move once the real conditions are known.
Getting Your Contract Reviewed Before You Sign
If you’re not sure whether the provisional sums in your contract or BOQ are reasonable — or whether too much of your project’s cost certainty has been given away — StructoTag can review your documents before you sign, and help you push back on vague or overused provisional allowances.
Frequently Asked Questions
Can a contractor make a profit on a provisional sum?
Typically yes — contracts usually allow the contractor to add their agreed overhead and profit percentage to the actual cost of work executed under a provisional sum, in the same way they would for the rest of the contract. This should be clarified in the contract terms rather than assumed.
Does a provisional sum count toward the total contract price?
Yes, the estimated provisional figure is included in the original contract sum, but it’s understood by both parties to be subject to adjustment once the actual cost is known.
Who decides how a provisional sum is eventually spent?
This is normally the client’s or their consultant’s decision, based on instructions issued once the work is properly defined — the contractor doesn’t have unilateral authority to spend a provisional sum however they choose.
Is it normal for a construction contract to have several provisional sums?
Yes, especially on residential projects where some design decisions (finishes, specialist installations, external works) are commonly finalized after the main contract is signed. The goal isn’t zero provisional sums, but keeping them limited to genuinely undefined items.
What happens if a provisional sum isn’t fully used?
If the actual cost of the work turns out to be lower than the provisional allowance, the difference is typically deducted from the contract sum at final account — the owner isn’t obligated to pay the full original provisional figure if the real cost came in lower.
Should provisional sums be listed separately from the main BOQ items, or mixed in with them?
Best practice is to list provisional sums clearly and separately within their relevant section of the BOQ, with a short description of what each one covers, rather than mixing them in indistinguishably with firmly priced items — this makes it much easier to track and reconcile them later.
Conclusion
A provisional sum is a normal and useful tool for handling genuine uncertainty in a construction contract — but it only works well when it’s used sparingly, described clearly, and properly reconciled against actual costs as the project progresses. As a building owner, your job isn’t to eliminate provisional sums entirely, but to make sure they’re limited to items that genuinely can’t be defined yet, and to keep a close eye on how they’re adjusted so your final account doesn’t come as a surprise.