Stop budget shock: prime cost and provisional sums over 15% in London
A prime cost sum is a supply-only allowance for an item you haven’t chosen yet, such as tiles or taps. A provisional sum covers work that can’t be designed or priced properly until the contract starts, such as drainage repairs, and includes both labour and materials. Both figures are placeholders. Your contractor reconciles them against actual cost once the real specification or scope is confirmed.
TL;DR:
Prime cost sums cover the supply of items you haven’t chosen yet and exclude installation costs, which are priced separately in the contract.
Provisional sums include both labour and materials for work that cannot be accurately designed or measured before the contract begins, often extending project timelines.
Defined provisional sums are detailed enough to include in planning, limiting contractor claims for delays or extra costs, unlike undefined sums which shift risks to homeowners.
Tracking all allowances as a percentage of total contract cost helps determine if a quote is fixed or just a range, especially if PC and PS sums exceed around 15%.
Converting allowances into fixed prices before signing the contract reduces surprises, with detailed supplier quotes and site surveys ensuring clarity and budget certainty.
Table of Contents
Prime cost vs provisional sum: the definitions that matter in your contract
A prime cost sum, often shortened to PC sum, is an allowance for goods or materials where you haven’t picked the exact product at tender stage. Think of a kitchen worktop or a bathroom suite priced at “allow £2,000” because you haven’t chosen the range yet. Crucially, that figure excludes fitting, labour, delivery, preliminaries, and your contractor’s overhead and profit. Those costs sit elsewhere in the contract, layered on top once you’ve made your choice.
A provisional sum, or PS, is broader. It’s used when a piece of work genuinely can’t be measured or designed properly before the contract is signed, covering both labour and materials together. Common examples include:
Drainage repairs discovered once floors are lifted
Asbestos removal where the extent isn’t known until surveys are done
Structural steel where ground conditions are still uncertain
RICS’ New Rules of Measurement sets out this distinction precisely, and it underpins how most JCT-based domestic building contracts in the UK are drafted.
How the two allowances actually compare
The practical differences between a PC sum and a PS become clearer once you line them up against how they behave in a real contract, from who specifies the item to what happens to your money if it’s never spent.
Feature | Prime cost sum | Provisional sum |
What it covers | Supply-only cost of a named item | Work not yet designed, including labour and materials |
Includes labour/installation? | No | Yes |
Who specifies the item | You (the homeowner), from a supplier range | The designer or contractor, once the scope is known |
When replaced by actual cost | Once you choose the product and get a supplier quote | Once the work is investigated, designed, and priced |
Effect on preliminaries/programme | Usually none, if fitting was already allowed for | Can extend the programme if the work is undefined and instructed late |
Once you’ve chosen your tiles or confirmed the drainage scope, your contractor should replace the allowance with the actual invoiced or quoted cost, and any unspent balance gets credited back to you rather than quietly absorbed into the final account.
Pro Tip: Add up every PC sum and provisional sum in your quote as a percentage of the total contract price. If they exceed around 15%, you don’t have a fixed price. You have a price range, and you should budget accordingly.
Defined vs undefined provisional sums: why the wording matters
Not all provisional sums carry the same risk, and this is where many homeowners get caught out. Under NRM rules used across most UK contracts, a provisional sum must be classed as either defined or undefined, and that single word changes who absorbs delay and extra cost.
A PS only counts as “defined” if the contract gives the contractor enough detail to plan around it, specifically:
The nature and construction of the work
How and where it’s located within the building
A reasonable estimate of quantity
Any limitations on how or when it can be carried out
If it’s defined, your contractor is deemed to have already allowed for it in their programme and preliminaries, meaning they can’t usually claim extra time or site costs when the work is instructed. If it’s undefined, that protection disappears, and the risk shifts onto you. Read your schedule of works carefully. If a provisional sum is listed with no supporting detail at all, ask your contractor to reclassify it or explain why it can’t be defined.
Who pays what and when: valuation and programme effects
Once a PS or PC sum is instructed, it stops being an estimate and becomes a real cost, valued against actual invoices or a pre-agreed quotation rather than a builder’s rough figure.
Your contractor obtains or submits a quotation for the actual work or supply item.
You or your contract administrator approve it in writing before work proceeds.
The original allowance is removed from the contract sum and the real cost inserted in its place.
Any difference, higher or lower, adjusts your final account accordingly.
This is where undefined provisional sums become expensive. If a PS for, say, unforeseen structural work is undefined and instructed midway through the build, your contractor can reasonably claim extra preliminaries and an extension of time, since they couldn’t have planned site costs around work they weren’t told about in advance. Contract administration guidance consistently flags this as one of the most common sources of budget overrun on domestic projects, particularly where surveys were skipped to save time before tender.
Checklist: reducing allowance-driven risk before you sign
Cost certainty on a refurbishment or extension comes largely from how much you resolve before the contract is signed, not after.
Finalise design decisions, tiles, sanitaryware, appliances, ironmongery, before tender wherever possible, so they become fixed prices rather than PC sums.
Insist every provisional sum in your schedule is labelled defined or undefined, and every PC sum is clearly marked supply-only.
Ask for a pre-instruction quotation on any provisional item before work starts, not an estimate after the fact.
Agree in writing how overheads, profit, and preliminaries apply once an allowance is instructed.
Keep a separate contingency fund. Don’t treat a provisional sum as your safety net, and insist on written credit for anything unspent.
Pro Tip: Before you agree a schedule of works, ask your contractor which line items are “buried” inside measured rates rather than shown separately. A tiling rate priced against a cheap tile assumption can hide a PC sum you never actually agreed to.
Tenen Ltd in practice: managing allowances on a London refurb
On a typical West London refurbishment, PC sums usually cover items like sanitaryware, ironmongery, and kitchen worktops, while provisional sums tend to surface once floors or walls are opened up, drainage, asbestos, or unexpected structural repairs. A project-managed approach documents every allowance in the schedule of works, obtains supplier quotations before instruction, and reconciles each one against the invoice rather than a verbal estimate. If you’d like allowances converted into fixed prices ahead of your project, a site survey is the natural starting point, as covered in our guide on budgeting a refurbishment.
When to accept allowances and when to insist on a fixed price
Allowances earn their place when a genuine unknown exists, drainage behind old plaster, ground conditions under an extension. They become a warning sign when a quote is stacked with undefined sums for things a proper survey could have priced. If budget certainty matters more to you than an early start date, finish your specification before tender. Where allowances remain unavoidable, document them tightly and price in a wider contingency, not a hopeful one.
— Mateja
Get allowances converted into fixed prices before you sign
Chasing down suppliers, defining provisional work, and reconciling invoices against a schedule of works is exactly the administrative grind that turns a straightforward refurbishment into a stressful one, as explained in this construction budgeting guide. Tenen Ltd’s director-led project management handles this from the first site visit, itemising every prime cost sum and provisional sum, requesting supplier quotations up front, and reconciling actual invoices so nothing gets buried in a measured rate.
This means fewer open-ended allowances by the time work starts on site, and a clearer final figure from day one. If your quote is carrying more provisional sums than you’re comfortable with, request a site survey for your full property refurbishment and get a written schedule of works before you commit to anything.
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FAQ
What’s the difference between prime cost and provisional sum?
A prime cost sum is a supply-only allowance for a specific item you haven’t chosen yet, such as sanitaryware, and excludes labour and fitting. A provisional sum covers work that can’t be fully designed or measured before the contract starts, and includes both labour and materials.
What is the difference between a PC sum and a PS item?
A PC sum only covers the cost of buying a named product, with installation priced separately elsewhere in the contract. A PS is a broader placeholder for an entire piece of work, labour and materials together, used when the scope genuinely isn’t known yet.
What does prime cost sum mean?
A prime cost sum means an allowance included in your quote for an item where you’ll choose the exact product later, such as taps or worktops. It’s a supply-only figure and never includes fitting, delivery, or your contractor’s overhead and profit, which are priced separately.
What is the difference between a provisional item and a provisional sum?
In practice, “provisional item” and “provisional sum” refer to the same thing: an allowance for work that can’t be accurately priced at tender stage. The key distinction that actually matters contractually is whether that sum is classed as defined or undefined, since that determines who bears programme and cost risk if it runs over.
Can I get money back if a provisional sum isn’t fully spent?
Yes. A provisional sum should only be spent against a written instruction and reconciled against actual invoices, not a builder’s estimate, and any unspent balance should be credited back to you in the final account.
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