Provisional Sums and PC Sums Explained for UK Builders

A provisional sum is an allowance for building work that cannot yet be fully specified or measured. A prime cost (PC) sum is an allowance for the supply of materials or goods that have not been finally selected. Both can appear in a building quotation, but they cover different uncertainties.

The important part is explaining what's included, how the price will be settled and how any increase or credit will be calculated.

Provisional sums vs PC sums: what's the difference?

A fixed-price item has an agreed price for a defined scope, subject to any lawful agreed changes. A contingency is a reserve for possible risks, not an allowance for an item awaiting specification.

For the wider distinction between firm quotations and approximate prices, see our guide to quotes vs estimates.

What is a provisional sum in construction?

A provisional sum is money included in a quotation or contract for anticipated work that cannot yet be described and priced accurately enough as a firm item.

On an extension, you might need to alter drainage but not know the existing pipe depth or route until further investigation. You could include £2,000 as a provisional sum for drainage alterations, alongside your assumptions and adjustment method.

Other examples include groundworks awaiting investigations or hidden structural repairs. Where a survey or better specification could settle the uncertainty before quoting, that may be preferable.

Don't use a provisional sum as a catch-all for site surprises: name the specific anticipated work. See also how to price a building job.

Defined and undefined provisional sums

The RICS New Rules of Measurement 2 (NRM 2) distinguish between two types of provisional sum when preparing bills of quantities:

• Defined provisional sum: The work isn't fully designed, but enough information is supplied about its nature, where and how it fits into the building, its approximate extent and any specific limitations.
• Undefined provisional sum: Some of that information is missing, so the work cannot be described to the required level.

Under these rules, a contractor is assumed to have allowed for defined provisional work in their programme, planning and preliminaries, but not for undefined work. Labelling an item “defined” does not make it so.

For a domestic building quote, clearly state what's known, what's uncertain and how the price will be agreed. If your contract uses JCT terms or formal measurement rules, follow that agreement; NRM 2 does not automatically govern every homeowner quote.

What is a PC sum (prime cost sum)?

A PC sum is an allowance for buying goods or materials where the precise selection is not yet known. Under RICS NRM 2 it is a supply-only allowance, commonly expressed as a rate per item or unit of material.

For instance, the customer may want a tiled bathroom but hasn't picked the tiles. Your quotation could allow £30 per m² for the tiles themselves. When they choose a product costing £42 per m², the supply allowance changes.

Other examples include sanitaryware, taps and door furniture. The PC sum is not the installed price.

What's excluded from the PC allowance?

Under the NRM 2 definition, the PC sum excludes fixing or installation, ancillary materials, and associated preliminaries, design costs, overheads and profit. Those costs need to be priced in the appropriate parts of the quotation or contract.

For tiles, show where you've priced adhesive, grout, trims, preparation and fitting. State whether delivery is included and check the purchase quantity, including cutting waste.

If the chosen tiles need different preparation or fitting, review those costs separately. See our bathroom renovation quoting guide.

Older contracts may also use “prime cost” for nominated suppliers or subcontractors, so check the contract's definitions.

Worked examples: how allowances change the final quote

The basic calculation is:

Revised total = original total − original allowance + agreed replacement value, plus or minus any separately agreed adjustments.

The figures below are illustrative and exclude VAT. For clarity, assume no extra percentage uplift and no changes to separately priced labour or other costs. Real projects may have different contractual provisions for overheads, profit, attendance and programme effects.

Example 1: a £2,000 provisional drainage sum increases to £2,700

You include £2,000 for anticipated drainage alterations. After inspection, the scope is confirmed and the replacement value is agreed at £2,700.

You replace the £2,000 allowance with £2,700. You don't add the full £2,700 to the original quote: that would count the allowance twice. Depending on the contract, the replacement may be valued using measured quantities, agreed rates or a separately agreed price.

Example 2: a tile PC sum increases from £600 to £840

The original allowance is for 20 m² of tiles to be purchased, at a supply-only price of £30 per m². The customer selects tiles priced at £42 per m². For this example, the order quantity stays at 20 m².

If the fitting requirements haven't changed, the separately priced fitting work can remain the same. If a different tile format affects labour, preparation or the quantity you need to order, work out and agree that effect separately.

Example 3: reconciling a £20,000 quote

Now put both adjustments into one building quotation:

The revised quotation is £940 higher: £700 for drainage plus £240 for tiles.

The calculation works in reverse. If drainage were valued at £1,700, the adjustment would be a £300 credit, subject to other agreed adjustments. Don't retain unused sums in the final bill.

How to include provisional and PC sums in a builder's quote

Each allowance should say what it's for, how much is allowed, what's excluded and how the final amount will be agreed.

Before you send the quote, check that:

1. The allowance is realistic. Base it on available measurements, supplier information and a plausible specification rather than an artificially low figure to make the headline price attractive.
2. The scope is named. “Drainage alterations subject to investigation” is more useful than “extra work as needed”, provided you also state the anticipated extent and assumptions.
3. Labour and extras are accounted for. For PC sums, identify fitting and sundries separately. For provisional sums, explain the anticipated scope and how other costs will be treated.
4. The adjustment process is clear. Say who approves the final selection or work, how the replacement value will be determined and when any change will be recorded.
5. The overall price is honest. Show which parts are fixed and which are allowances. A total containing significant provisional amounts shouldn't be presented as wholly fixed.

In Quote Forge, you can label an item as a provisional sum or PC allowance and use its description to explain what's included. Add the adjustment basis in your quotation notes so the customer can see how the allowance will be handled.

Use the descriptions and notes in your building quotation to make the distinction visible to the customer, not hidden in small print.

Example wording: provisional sum for drainage

Drainage alterations — provisional sum: £2,000 excluding VAT. This allowance is for anticipated alterations to the existing drainage serving the extension, including excavation, pipework, backfilling and disposal within the agreed working area. The existing pipe route, depth and condition have not yet been confirmed. Reinstatement of finished paving and any statutory charges are excluded. After inspection, we will provide a description and price for the confirmed work and seek written agreement before proceeding. The original £2,000 allowance will be deducted and replaced by the agreed value. Any effect on the completion date will also be discussed and recorded.

This is illustrative wording, not a complete contract term. Add known dimensions and access assumptions, and follow any formal instruction procedure in the actual contract.

Example wording: PC allowance for floor tiles

Floor tile supply — PC allowance: 20 m² at £30 per m² = £600 excluding VAT. This covers the supply of 20 m² of floor tiles only. Labour for fitting and the specified adhesive, grout and trims are included in separate quotation items. Delivery and any further quantity required beyond the stated 20 m² are not included in this allowance and will be priced and agreed before ordering. Once the tiles have been selected, we will confirm the supply price and adjust the £600 allowance up or down by the agreed difference. Any change to the fitting requirements will be quoted separately.

Check that 20 m² is an appropriate purchase quantity, allowing for cuts and waste. State clearly where excluded costs appear elsewhere in the quote.

How to manage allowance adjustments during the job

Record each adjustment when the decision is made, not just at final invoice.

Before committing to the revised work or purchase, follow the contract's instruction and approval procedure. For a straightforward domestic job, good practice is to send a written breakdown of the proposed replacement value, the difference from the allowance and any likely effect on timing, then record the customer's decision. Some formal contracts prescribe different valuation and notice procedures.

Keep an allowance adjustment record simple enough to use on site:

Also record the date, supporting costs, customer approval and any programme effect. Mark unagreed figures proposed. RICS final account guidance favours settling provisional sums during the job, not at the end.

An allowance adjustment isn't always the same as extra work

Replacing the price of work already described differs from a customer-requested change of scope. The expected drain diversion is not the same as a later request for drainage to a new outbuilding.

Keep those changes separate. Our guide to pricing extra building work covers that distinction in more detail.

Be especially clear when quoting for homeowners

Under UK consumer law, contract terms must be fair and transparent. The Competition and Markets Authority warns that a term giving a trader an unrestricted ability to increase the price may be unfair. A labelled allowance is not permission to charge whatever you like later. The original quote, the wording of the adjustment mechanism and what the customer agreed all matter. See the CMA's guidance on fair consumer contracts.

Citizens Advice explains the distinction between quotations and estimates when homeowners challenge unexpected charges. Clear figures and written agreement are safer than arguments afterwards.

The examples exclude VAT for clarity. On a real quote, show the VAT treatment and total payable where applicable. Construction VAT varies by circumstances: see HMRC's VAT Notice 708.

Common mistakes with provisional sums and PC sums

Putting in unrealistically low allowances. A low PC tile rate or provisional groundworks figure makes a quote look more attractive but can create a predictable increase later. Use a sensible basis and make the uncertainty visible.

Leaving out fitting, delivery or sundries. A PC sum isn't an installed price. Show where these costs appear and avoid presenting them as unexpected extras when they were foreseeable.

Adding overheads or profit twice. Under NRM 2, the treatment of percentage additions is explicit, but not every domestic quote follows NRM 2. Identify what you have priced elsewhere and what, if anything, is separately agreed on adjustments. Don't confuse markup with margin.

Forgetting credits. If an item comes in below its allowance, calculate and communicate the reduction using the agreed pricing method, just as you would an increase.

Calling the whole price fixed. Say which parts are fixed and which are allowances, including in the total-price summary.

Frequently asked questions

Is there a standard percentage for a provisional sum?

No universal percentage makes a provisional sum suitable for every job. It should relate to identifiable anticipated work, supported by the information available. Don't confuse the allowance itself with a separately agreed percentage for overheads and profit.

Is a provisional sum a maximum price?

Not normally. It is an allowance that may go up or down under the agreed terms, rather than automatically being a cap. If you intend a maximum figure, spell out what is covered and what happens if that limit is approached.

What if the customer supplies the tiles?

Agree what supply allowance is being removed or credited, then confirm responsibility for ordering, delivery, defects, damage, quantities and suitability. The fitting price may also need reviewing if the supplied tiles differ from those assumed.

Can you show allowances on a PDF quotation?

Yes. Label them as provisional sums or PC allowances, include their descriptions and make sure the quoted total counts each amount once. A professional PDF makes the breakdown easier to read, but it doesn't replace an agreed adjustment process.

Keep allowances clear from the outset

Provisional sums deal with uncertain work; PC sums deal with uncertain material or goods supply. Use realistic figures, explain exactly what each allowance covers, and show every later increase or credit against the original amount.

Quote Forge helps UK tradespeople prepare itemised, branded quotations and download them as PDFs. You can describe an allowance in a quotation item, while keeping the supporting cost breakdowns and customer approvals in your own job records. That gives customers a clearer starting point without implying that the initial allowance is a guaranteed final price.