The Gap Between Tender Sum and Final Account

Introduction

One of the most common sources of financial stress and conflict in domestic building contracts is the gap between the contractor's tender price — agreed before construction starts — and the final account price — the total paid by the time the building is complete. In residential renovation and extension projects, the final account routinely exceeds the tender sum by 10–25%, and in complex or poorly managed projects the gap can be larger. Understanding why this happens, what the legitimate sources of additional cost are, and how to limit the gap is one of the most important aspects of financial management for a homeowner undertaking a building project.

Why the Final Account Exceeds the Tender Sum

Variations

A variation is an instruction from the Contract Administrator (CA) or the homeowner to change the scope of works from the original contract documents. Variations are the most common and legitimate source of post-contract cost increase. They include:

• Design development changes: Changes to the specification or detail that arise during construction — a different tile, a relocated door, additional shelving, a changed kitchen layout
• Client changes: Scope additions instructed by the homeowner — adding items not in the original contract, upgrading specification, adding new elements
• Unforeseen conditions: Works required as a result of conditions discovered during construction that were not known at tender — typically draws on the contingency and is formalised as a variation
• Architect's instructions: Instructions from the architect clarifying or amplifying the original design — where the original drawings were ambiguous or incomplete

Provisional Sum Adjustments

Provisional sums are cost allowances in the contract for items whose exact cost cannot be determined at tender. Common provisional sum items include:

• Groundwork and foundation works (subject to ground conditions found during excavation)
• Specialist subcontractor works where detailed scope is not yet defined at tender
• External works and landscaping
• Builder's work in connection with specialist trades

Provisional sums are adjusted to actual cost when the relevant work is instructed or completed. Where the provisional allowances in the tender were set too low — either through optimistic estimation or because the specification was not complete at tender — the adjustment will increase the final account.

Loss and Expense

Loss and expense is a claim by the contractor for additional cost arising from relevant events that have caused delay or disruption — for which the employer (homeowner) is responsible under the contract. Common loss and expense events include:

• Late provision of information required to carry out the works (architect's drawings, specification, decisions)
• Employer's instructions causing delay or disruption
• Opening up of works for inspection where the work is found to be satisfactory

Loss and expense claims are a legitimate contractual mechanism but are often contested. Poorly managed information flow, late architect's instructions, and frequent employer changes are the most common causes of loss and expense claims in domestic contracts.

What a Typical Gap Looks Like

How to Limit the Gap

Complete Design Before Tender

The single most effective measure is ensuring that the design package is fully complete before tenders are invited. Every provisional sum, every undefined element, and every design issue left open at tender is a source of post-contract cost increase. A well-developed technical design package at RIBA Stage 4 — complete drawings, specification, and schedules — produces a tender sum that closely reflects the final account cost.

Discipline Around Client Changes

Client changes during construction are the most controllable source of cost increase. Every change instruction should be subject to a clear process — the contractor provides a quotation, the CA assesses it, and the homeowner approves the cost before the instruction is issued. An informal instruction culture — where changes are verbally agreed on site without formal pricing — is a common cause of final account disputes.

Realistic Provisional Sums

Where provisional sums cannot be avoided (for items genuinely unknown at tender), they should be set at realistic rather than optimistic levels. An overly low provisional sum may produce a competitive-looking tender but guarantees a final account overage. A quantity surveyor reviewing provisional sums before the tender package is issued can identify and correct unrealistic allowances.

Information Management

Late information — architect's details released after the contractor needs them — causes disruption and provides grounds for loss and expense claims. Managing the information release programme (the Information Required Schedule) ensures that the contractor has all necessary design information in advance of when it is needed on site.

The Final Certificate

The final account is formally resolved in the Final Certificate — issued by the CA after the Making Good Certificate, taking account of all variations, provisional sum adjustments, and loss and expense agreed during the project. The Final Certificate records the final contract sum and the balance due (either to the contractor or a credit to the employer). The Final Certificate is conclusive evidence of the final contract sum in the absence of proceedings challenging it. Understanding the Final Certificate process helps homeowners manage the financial close of the project — see our guide on the Making Good Certificate for the preceding stage.

Conclusion

A gap between the tender sum and the final account is normal and expected in a domestic building contract — the question is how large the gap is and whether it is genuinely due to unforeseen conditions or to poor change management. A homeowner who maintains discipline around client changes, insists on formal quotation and approval before all instructions, and sets aside adequate contingency for genuinely unforeseen conditions should see a final account that is 10–15% above the tender sum at most. A homeowner who changes the design frequently during construction, instructs works verbally without pricing, and has no contingency reserve risks a final account 30–40% above the tender sum and significant financial stress. An experienced architect managing a contract with rigorous financial discipline will keep the gap as small as possible and ensure there are no surprises at final account.