Why does my contract have provisional sums?

Indicates incomplete design or unknown site conditions at signing. Common in heritage (uncertainties under finishes), basements (ground conditions), rewires (cable routing). Reduce by completing structural, electrical, asbestos surveys before tender; architect completing detailed design; exploratory opening pre-contract. Some inevitable; minimise to reduce variation risk.

Can contractor exceed without my approval?

Depends on contract. JCT Minor Works and Intermediate require contractor obtain client/architect instruction before exceeding. Smaller bespoke contracts may not have this protection — verify before signing. Best practice: clause requiring client sign-off on any provisional sum overrun above 10–20% threshold.

How is margin calculated?

Contractor typically charges 7.5–15% margin on PC sum variance — covers admin, ordering, handling. PC £4,500 actual £5,800 variance +£1,300; margin at 10% = £130 on variance. Total +£1,430. Margin negotiated at contract; typical 10–12.5% domestic JCT. Verify margin %; check if applied to full PC value or variance only.

Can I avoid PC sums entirely?

Yes if you select all products pre-contract — full FF&E schedule with model numbers, prices, supplier confirmation, lead times. Practical only for clients committing early or simple finish palettes. Most renovations have £15–35k of PC sums for sanitary ware, lighting, ironmongery, decorating — selection benefits from client during build when design coherence visible.

Provisional Sums vs Prime Cost London 2026 | Builderr

Provisional Sums vs Prime Cost Sums in London Renovation Contracts

Prime Cost (PC) sums cover items specified but not yet selected (e.g. £6,000 PC for kitchen taps — model TBC). Provisional sums cover work where scope or cost is uncertain (e.g. £8,000 for unknown foundation depth). Both reconcile against actual cost at final account; over/under variations re-priced. Typical PC sums 8–15% of contract; provisional sums 5–12%. High provisional sums signal incomplete design — minimise both.

PC sum mechanics

PC sum: included for items specified by category but exact model not chosen at contract date. Example: PC £4,500 for sanitary ware — basins, WC, shower, bath — exact brand selected during construction. Allows client time to make selections without delaying contract. Reconciliation at final account: actual cost replaces PC; difference adjusts contract sum. PC £4,500 actual £5,800 → contract sum +£1,300 plus typically 10–15% contractor margin on variance £130–£195. PC includes supply only — contractor labour to install is main contract. Risk: client over-specifies during selection. Mitigation: realistic mid-market PC sum; specify upper limit; require sign-off on selections vs PC budget.

Provisional sum mechanics

Provisional sum: work cannot be defined precisely at contract — scope or extent uncertain. Example: provisional £6,000 for underpinning depth only knowable post-excavation. Two types under JCT 2016: (a) Defined — scope reasonably specified, cost uncertain; contractor includes prelims and programme in main contract. (b) Undefined — scope and cost both uncertain; contractor entitled to additional prelims and programme extension when scope confirmed. Most domestic JCT use defined. Reconciliation: actual work substitutes provisional sum; contractor entitled to margin (7.5–15%) plus reasonable prelims. Provisional £6,000 actual £9,200 → +£3,200 plus margin £240–£480 plus prelim extension. Higher-risk than PC sums.

When to use each

PC sums appropriate for: sanitary ware, lighting, kitchen appliances, ironmongery, decorating finishes — selection needs client showrooms but install method standard. NOT appropriate for: structural works, services first-fix, specialist trades — need full design before contract. Provisional sums appropriate for: unknown ground conditions (foundations, drainage), specialist surveys incomplete (asbestos abatement TBC), exploratory opening up (electrical rewire scope), heritage discoveries. NOT appropriate for: standard works where scope is knowable — should be measured into main contract. Red flag: contracts with >15% provisional sums signal incomplete design and high variation risk.

Reconciliation and disputes

Final account reconciles PC and provisional sums to actual cost. Process: contractor submits supplier invoices for PC items and time-sheets/measurements for provisional sums; QS verifies; net adjustment to contract sum; agreed VO. Disputes: contractor claims £8,500 for provisional £6,000 underpinning citing additional depth; client disputes citing scope was clear. Resolution: site investigation report, engineer variation, documented evidence; if unresolved, JCT adjudication. Minimise disputes: realistic PC sums, complete surveys pre-contract, contractor must allow client to challenge cost before commitment, ceiling on provisional sums above which client re-approves.