Avoiding Cost Overruns in London Building Projects: A Homeowner's Guide 2025

Cost overruns in London building projects occur for several distinct reasons, each with a different prevention approach: Incomplete scope and design at the time of pricing: the most common cause of cost overruns. Where a contractor is asked to price a project based on limited information — a rough brief, planning drawings only (not construction detail drawings), or a verbal description of the work — the price will inevitably change as the real scope emerges. What was a £75,000 rear extension priced on planning drawings can become a £100,000+ project once the structural engineer's design, the drainage detail, the Building Control requirements, the specification of the bi-fold doors, and the kitchen selection are all resolved. Prevention: resolve as much of the design and specification detail as possible before inviting contractors to price. Use construction drawings (not just planning drawings) to obtain a price. Provisional sums: where some elements cannot be designed or priced until work is opened up (structural conditions revealed when an old wall is removed; drainage conditions revealed when the garden is excavated), provisional sums (PS) are included in the contract price as a best estimate. If the actual cost of the PS item turns out to be higher than the PS allowance, the contract price increases. Prevention: understand and review the PS items in the contract. Query unrealistically low PS figures. Include adequate contingency for PS items that are uncertain (see below). Scope creep and variations: additional work items instructed during the project that were not in the original contract scope. Some variation is inevitable — unforeseen defects uncovered at strip-out, structural issues revealed when opening up, and client-initiated changes to specification are all common. But a poorly controlled variation process (verbal instructions, no agreed price before work proceeds, retrospective pricing disputes) is a major source of budget overrun. Prevention: require all variations to be instructed and priced in writing before work proceeds. Use a formal variation instruction process in the contract. Client-initiated scope changes are the client's responsibility — but they should be priced and approved before the contractor proceeds. Hidden defects and unforeseen site conditions: London's Victorian and Edwardian building stock regularly conceals defects that were not visible or predictable before work started: concealed damp (behind tiles, within wall cavities); old or damaged drainage; inadequate or poorly specified structural elements (old steel beams that do not meet current loading requirements); embedded services (old gas pipes, water supply pipes in positions not shown on any available record); asbestos (particularly in 1930s-1970s London construction). Prevention: commission a thorough pre-construction survey before pricing (not just a mortgage lender's valuation — a full RICS homebuyer survey or building survey). Include a realistic PS and contingency for unforeseen conditions appropriate to the age and condition of the property. Unrealistic initial pricing: where a contractor prices low to win the work and prices variations high to make the margin back. This is a known practice in the London contractor market. Prevention: use properly vetted contractors (FMB, TrustMark, referenced firms with verifiable track records on similar projects). Compare quotes carefully — an unusually low headline price should prompt investigation of what is excluded and what the provisional sum strategy is, rather than celebration.

A project contingency is a budget reserve held by the client (not the contractor) to cover unforeseen cost increases that are outside the fixed contract scope. The contingency is separate from the contract sum and does not appear in the contractor's price. Recommended contingency levels for London building projects (2025): New-build construction or full extension on a cleared site: 5-8% of the contract sum. Medium-complexity refurbishment on a post-1950s property in generally known condition: 8-12% of the contract sum. Full refurbishment of a Victorian or Edwardian property, particularly where services and structure are original and in unknown condition: 12-20% of the contract sum. Full refurbishment plus extension of a Victorian property: 15-20% of the contract sum. Properties with known complications (basement excavation, party wall issues, heritage features, proximity to mature trees, suspected asbestos or contamination): 20-25% of the contract sum. Examples: for a £90,000 rear extension and kitchen refurbishment on a 1900 Victorian terrace: recommended contingency = 15% = £13,500. Hold this as a reserve in the project budget and only commit it when a specific unforeseen cost arises. For a £50,000 bathroom and bedroom refurbishment on a 1960s purpose-built flat: recommended contingency = 10% = £5,000. How to use contingency correctly: the contingency is not part of the budget handed to the contractor. It is held by the client and released only when a specific, agreed variation instruction is raised. Contractors should not know the size of the client's contingency (if they do, they may be less motivated to find cost-efficient solutions to unforeseen problems). Where the contingency is not used by the end of the project, it is returned to the client's savings. A project that completes within budget and does not need the contingency is a successful outcome of good pre-construction planning — not a sign that the contingency was set too high.

The right contract structure significantly reduces the risk of a London building project going over budget: Fixed-price contract: where the scope is well enough defined to support it (construction drawings, specification, and material selections are complete before pricing), a fixed-price contract protects the client against cost increases on the defined scope. The contractor cannot increase the contract sum except through a formal, agreed, client-authorised variation instruction. For a rear extension with a properly specified scope, a fixed-price JCT Homeowner Contract (HO/B 2016) or similar standard form provides strong budget protection. Provisional sums: where some elements cannot be fixed (structural element conditions, drainage, or specification of client-procured items not yet selected), provisional sums should be clearly identified in the contract. The PS is replaced by the actual cost when the work is instructed and completed. To protect budget: challenge PS items that appear unrealistically low (a plumber quoted at £1,000 PS for drainage alterations that may actually cost £3,000 is a risk to budget); include contingency above the PS items; and resolve specifications early to eliminate PS items wherever possible. Stage payments linked to milestones: a well-structured payment schedule (as discussed in the contractor appointment guide) pays the contractor in line with actual progress. This prevents the client paying ahead of progress (over-certifying) — which weakens the client's commercial position in any dispute. Retention: a retention of 2.5-5% of the contract sum held for 6-12 months after practical completion protects the client against defects. The retention is the contractor's financial incentive to return and address defects quickly. Variation instruction protocol: a formal requirement in the contract that all variations are instructed in writing, priced in advance, and approved by the client before the work proceeds. Verbal instructions during a site visit — "while you're at it, can you also..." — should always be followed up with a written variation instruction that captures the agreed price. Without a written agreement, the contractor's retrospective valuation of the verbal instruction will be the only basis for the variation cost — and it will almost certainly be higher than the client expected.