What Is a Prime Cost Sum in Construction?
A prime cost sum (PC sum) is an allowance in a building contract for a specific item where the client or architect has not yet selected a supplier or product. The contractor includes the PC sum as a placeholder; once the item is chosen, the contract is adjusted — upward if the actual cost exceeds the allowance, downward if it comes in below it.
PC sums appear frequently on fit-out and renovation projects where the client wants to retain design control over specific elements — sanitaryware, kitchen units, tiles, light fittings — without holding up the quoting and contracting process while selections are made.
How a Prime Cost Sum Works in Practice
Here's a concrete example. A contractor quotes for a bathroom renovation and includes:
"PC sum — bathroom wall tiles — £600 supply only. Client to select product. Tiling labour is included in the schedule at the quantities shown. Any difference between the PC sum and actual tile cost will be adjusted by variation."
What happens next:
- The client visits a tile showroom and selects tiles at £14 per square metre — total tile cost for the bathroom: £420. The contractor credits £180 back to the client.
- Or: The client falls in love with handmade tiles at £28 per square metre — total cost £840. The contractor raises a variation for £240 and the contract sum increases.
- Or: The client selects tiles exactly at the PC sum. No adjustment needed.
The PC sum allows the contract to be placed, the work to be programmed, and the trades to be booked — all without waiting for every product selection to be finalised.
Items Commonly Covered by PC Sums
On UK domestic renovation and new build projects, prime cost sums are typically used for:
| Item | Typical PC Sum Range | Notes |
|---|---|---|
| Sanitaryware (WC, basin, bath) | £400–£2,000 per bathroom | Hugely variable — entry-level to designer |
| Kitchen units and appliances | £3,000–£15,000+ | The most significant PC sum on most projects |
| Floor tiles (supply only) | £600–£2,500 | Varies by area and product specification |
| Wall tiles (supply only) | £400–£1,800 | Per room; bathroom and en suite typically separate |
| Light fittings | £500–£3,000 | Client selection; electrician installs |
| Ironmongery (handles, hinges) | £200–£1,000 | Finish choice — polished chrome, brushed brass, etc. |
The key risk for clients is that PC sums are sometimes set unrealistically low — particularly on kitchen units — to make the overall quote look competitive. When you receive a quote with PC sums, check that the allowances are actually achievable for the quality level you're expecting.
PC Sum vs Provisional Sum — the Key Distinction
The two terms are often confused. Here's the clearest way to remember the difference:
- Prime cost sum: The item is known. The scope is known. The price is not yet known because the client hasn't chosen the specific product or supplier. "We know you're having tiles in the bathroom — we just don't know which ones yet."
- Provisional sum: The scope itself is uncertain. "We know there may be drainage work needed — we won't know the full extent until we excavate."
Both are placeholders that get replaced by actual costs during the project. But they arise from fundamentally different circumstances: one from a client choice not yet made, the other from a scope of work not yet defined.
See also: What Is a Provisional Sum? — the related article covering provisional sums in full.
For further context on how these items appear in a complete quote document, see: Building Quote Sample Format.
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Try RenoCalc FreeFrequently Asked Questions
What are typical examples of prime cost sums on a building project?
Common examples include sanitaryware (WC, basin, bath — client to choose range), kitchen units and appliances (client to select supplier and specification), floor and wall tiles (client to select product), light fittings, and ironmongery such as door handles. PC sums are most common on renovation and fit-out projects where the client wants design control over specific elements without delaying the contract being placed.
What happens if the client chooses something more expensive than the PC sum?
The difference is added to the contract sum as a variation. If the PC sum was £600 for floor tiles and the client selects tiles costing £950, a variation for £350 is raised. The contractor should always confirm the actual cost against the PC sum before ordering, giving the client advance notice of any variation before it becomes a commitment.
What is the difference between a PC sum and a provisional sum?
A prime cost sum is for a specific, known item where the scope is clear but the product hasn't been chosen — "PC sum £600 for floor tiles." A provisional sum is for work where the scope itself is uncertain — "Provisional sum £2,500 for drainage repairs." PC sums vary because the client makes a product selection; provisional sums vary because the extent of work becomes clear once the project is underway.
How is a PC sum adjusted during a building project?
Once the client selects the specific product or supplier, the contractor obtains the actual cost. A variation is raised removing the PC sum and replacing it with the actual supply cost. If the actual cost is higher, the contract sum increases by the difference. If lower, the contract sum is credited. The contractor's installation or fitting labour remains as originally priced — only the supply cost of the nominated item changes.
Can the contractor refuse to use the client's chosen product?
The contractor cannot unreasonably refuse a client's product selection against a PC sum — but they can raise practical concerns, such as if the product is incompatible with the planned installation method, requires specialist fitting skills, or has a significantly longer lead time that would affect the programme. Any such concerns should be raised promptly and in writing before the client commits to the selection.
What JCT contract provisions cover PC sums?
Under JCT Standard Building Contracts, prime cost sums are formally called "Provisional Sums" in some editions, with nominated subcontractors and nominated suppliers managing the supply of specific items. In practice on domestic projects, PC sums are handled informally as allowances in the quote. For detailed JCT contract guidance, see: JCT Ltd — Joint Contracts Tribunal.
