Somewhere in a UK renovation quote there is a line that reads like a price and is not one. It is headed provisional sums, PC sum, or allowance, and carries a confident round number: £2,500 for drainage, £4,000 for kitchen units. Every other line was measured. That one was estimated, and the gap is funded by you.
Builders are not being dishonest when they use them. A provisional sum is the correct instrument for work that genuinely cannot be priced before the job starts, because nobody can quote a foundation depth before the trial hole is dug. The failure is one of translation. The homeowner reads a price. The industry wrote a placeholder — and the standard behind it has already decided which of you carries the delay when the real number arrives.
That standard is the RICS New Rules of Measurement, and it draws a line almost no consumer-facing page mentions: every provisional sum is either defined or undefined. It decides whether your builder was ever expected to allow for that work in the programme, and therefore whether instructing it buys them extra time and site costs on top.
A provisional sum is not a price. It is the builder writing down, in advance, that this part of the job has not been priced.
What follows is how the UK industry defines these lines, what a prime cost sum is and is not, and the questions that turn a placeholder into something a prepared homeowner can hold a builder to.
What are provisional sums on a UK renovation quote
The RICS New Rules of Measurement (NRM2) defines a provisional sum as a sum of money set aside to carry out work that cannot be described and given in quantified items in accordance with the tabulated rules of measurement. In plain English: the work will happen, but when the quote was written it could not be measured, so a number was reserved instead.
NRM2 is the RICS standard for measuring building works in the UK. It took effect on 1 January 2013, replaced the older Standard Method of Measurement, and is what quantity surveyors and the better small builders price to. It matters to a homeowner because the rules it sets are not the rules most people assume.
Two of them change how you read a quote. First, NRM2 requires every provisional sum to be identified as either defined or undefined. Second, provisional sums are stated exclusive of overheads and profit. The builder's margin is not inside that £2,500 for drainage. It is added when the work is valued, so the line grows for two reasons at once.
None of this is hidden. It is written for a professional audience, and a homeowner who has not learned to read a quote line by line assumes the total at the bottom is the total.
What is the difference between a defined and an undefined provisional sum
Under NRM2 clause 2.9.1.2, a provisional sum for defined work is one where four pieces of information have been supplied even though the work is not fully designed: the nature and construction of the work; how and where it is fixed to the building and what other work is fixed to it; a quantity indicating scope and extent; and any specific limitations.
Clause 2.9.1.3 sets out the consequence. Where a provisional sum is given for defined work, the contractor is deemed to have made due allowance for it in their programming, planning and pricing of preliminaries. The time is already in the programme. The site running costs are already in the price. Instructing the work does not, on its own, entitle them to more of either.
Clause 2.9.1.4 does the reverse. Where any of that information cannot be given, the work must be described as an undefined provisional sum, and the contractor is deemed not to have made any allowance in programming, planning and pricing preliminaries. Instruct one and you may be paying for the work, for the weeks it adds to the programme, and for the scaffolding, welfare and site management that come with them.
Clause 2.9.1.5 closes the loophole: a sum labelled defined that does not carry those four pieces of information is construed as undefined regardless of the label. The information behind the word decides it, not the word.
For every provisional sum, ask in writing: is this defined or undefined, and if defined, what nature, fixing, quantity and limitations have you assumed?
A builder who answers has handed you a specification you can price against a second quote. One who cannot has told you the programme risk is sitting with you.
One caveat. NRM2 is a measurement standard, not consumer law, and it bites hardest where the contract expressly adopts the RICS measurement rules — which the standard JCT forms do. On a small domestic job with no bill of quantities the labels may not appear at all. That absence is the finding. Ask anyway.
Know the real number before the quote arrives
The free Renovation Cost Calculator gives you a trade-by-trade estimate in under 5 minutes — before your first trade conversation. It is the benchmark that shows you which provisional sums on a quote are set low.
How is a prime cost sum different from a provisional sum
The two are routinely used as if they were the same thing, and the difference costs money. NRM2 defines a prime cost sum, usually written PC sum, as money included in a unit rate to be spent on materials or goods from suppliers — supply-only ceramic wall tiles at £50 per square metre, or supply-only door furniture at £120 per door. It is a supply-only rate for goods whose precise quality is not yet known.
The exclusions are the whole point. A PC sum excludes all costs associated with fixing or installation, the ancillary and sundry materials needed to fix the goods, and both the subcontractor's and the main contractor's design fees, preliminaries, overheads and profit.
So a £3,000 PC sum against taps and sanitaryware is £3,000 of goods at the merchant. The plumber fitting them is not in it. The seals, brackets and waste are not in it. The builder's margin is not in it. A homeowner who picks £3,000 of brassware and believes the bathroom budget is intact has misread the line by the cost of an entire trade.
The test is the same for both: a provisional sum reserves money for work, a prime cost sum reserves money for goods, and neither reserves money for the finished result. Some builders still use PC sum loosely to mean any fixture allowance, which is why the answer has to come in writing.
Which provisional sums turn up on almost every UK quote
On an extension, a loft conversion or a refurbishment, the same lines carry the same unknowns. Recognising them is how you decide which to close before signing.
- Groundworks and foundations. Depth is unknown until a trial hole is dug, so the sum covers what is found in the ground rather than what is drawn, and it is the line most likely to double.
- Drainage diversions and connections. Existing runs are rarely surveyed before a quote is issued, and a build-over agreement with the water company can change the route after the price is set.
- Structural steelwork and opening formation. Beam size follows the engineer's calculation, which normally lands after the quote, so steel, padstones and fire protection are all carried as a reserve.
- Electrical alterations and consumer unit works. A Part P competent person can only see the existing installation once it is opened up, and an older board may not carry the new circuits.
- Making good, plastering and decoration. The extent depends on how much of the existing finish survives demolition, which is why this line is almost always an allowance and almost always low.
- Kitchen and bathroom fittings. The hob, the worktop, the taps and the sanitaryware are carried as a sum because the homeowner has not chosen them yet, and unchosen items cannot be priced.
A seventh line differs across the UK: any sum covering the works required by building control. In England and Wales that means Building Regulations approval through local authority building control or a registered building control approver. In Scotland it is a building warrant, granted before work starts and closed with a completion certificate. In Northern Ireland building control sits with the district council.
What happens to the price when a provisional sum is instructed
When the work behind the sum is instructed, the sum is removed from the contract price and replaced by a valuation of the work actually carried out. In principle the contract sum can move down as well as up. In practice it moves up, for reasons that have nothing to do with dishonesty.
By the time a sum is instructed, the trades are on site, the scaffold is up, and there is no second quote in the room. Every commercial pressure that produced a sharp price at tender has gone. The work is valued as a variation, overheads and profit are added on top because NRM2 kept them out of the original sum, and if the sum was undefined the programme consequence is a separate conversation again.
This is also where the payment schedule quietly breaks. If your stage payment points were set against the original contract sum, instructed work has to be written into that schedule deliberately, or you pay out of sequence and lose the leverage the final stage exists to protect. Work valued out of a provisional sum still belongs on the snagging list.
Provisional sums are one of the four or five mechanisms that quietly explain why UK renovations go over budget even when nobody on site has done anything wrong.
How should a homeowner answer a quote full of placeholders
Not by refusing them. A quote with no provisional sums on a project with genuine unknowns is either a builder who has not looked, or one who has loaded the contingency invisibly into the measured rates. The prepared homeowner shrinks the provisional portion of the price before signing, and documents what is left.
Total every provisional and prime cost sum and express it as a share of the contract sum. If a tenth or more of the price is provisional, you are not holding a fixed price. You are holding an estimate with a fixed-looking cover on it.
Then convert what can be converted. Choosing the hob, the worktop, the taps and the sanitaryware before quotes go out turns the largest allowance on the page into a measured price. Commissioning trial holes and a drainage survey before contract does the same for the ground. Each conversion moves a line out of the unknown column, which is the work a written scope of works exists to do. Specification and contract review are Phases 3 and 6 of The 12-Phase System, and provisional sums are the clearest evidence of a project reaching Phase 6 with Phase 3 unfinished.
For anything left, get the defined or undefined answer in writing and agree how instructed work will be valued.
See the Renovation Blueprint systems
Every room. Every phase. Every decision — documented before it has to be made.
If the cost baseline is the first step, use the free Renovation Cost Calculator — a trade-by-trade estimate in under 5 minutes, before any builder has quoted.
Frequently asked questions
What are provisional sums in a UK building quote?
A provisional sum is money set aside in a quote for work that could not be described and measured when the price was prepared. RICS NRM2 defines it that way, requires it to be labelled defined or undefined, and states that it excludes the builder's overheads and profit. When the work is instructed the sum is removed and replaced by a valuation of what was done.
Is a provisional sum included in a fixed price contract?
It sits inside the contract sum, but it is not fixed. Every provisional sum is an amount both parties have agreed to revisit once the work can be measured. The fixed portion of a fixed price is only the measured portion, so a quote carrying a tenth or more of its value in provisional sums should be read as an estimate.
What is the difference between a defined and an undefined provisional sum?
A defined sum comes with the nature of the work, how and where it is fixed, a quantity showing scope, and any limitations. Because that information exists, NRM2 deems the contractor to have allowed for the work in their programme and preliminaries. An undefined sum lacks some of it, and the contractor is deemed not to have allowed at all, so instructing it can add time and site costs on top.
Does a prime cost sum include installation?
No. Under NRM2 a prime cost sum is a supply-only rate for materials or goods from a supplier. It excludes fixing and installation, the ancillary materials needed to fix the goods, and both the subcontractor's and the main contractor's design fees, preliminaries, overheads and profit. A PC sum for taps covers the items at the merchant and nothing beyond.
Can a builder increase a provisional sum without asking me?
The sum is replaced by a valuation of the work done, so the figure moves as a matter of course rather than by permission. The process is what you control. Agree in writing before signing that instructed work is priced and approved as a variation before it proceeds, that the revised contract sum is confirmed, and that the stage payment schedule is updated to match.
Are provisional sums treated the same across England, Scotland, Wales and Northern Ireland?
The measurement standard is the same UK-wide, because NRM2 is a RICS standard rather than legislation. What differs is the approval regime a sum for statutory works must satisfy. England and Wales use Building Regulations approval through building control. Scotland requires a building warrant before work starts and a completion certificate at the end. Northern Ireland works through district council building control.