Construction Contingency Guide for London Home Projects

Every construction project encounters unforeseen conditions. Buildings are complex, ground conditions are unpredictable, and designs evolve. A project without contingency has no buffer against the inevitable surprises.

Running out of money mid-build is one of the most stressful things that can happen to a homeowner. Work stops, trades leave site, and the costs of delay and restarting can exceed what the contingency would have cost.

Contingency is not pessimism. It is professional project planning.

The right level of contingency depends on the type and complexity of the project, and how much information is available at the point of pricing.

New build or conversion projects (high complexity, more unknowns): Allow 15–20% of the build cost.

Major refurbishment of older properties: Allow 15% of the build cost. Victorian and Edwardian London terraces are particularly prone to hidden issues: old drainage, failed DPCs, unknown structural modifications, asbestos, and substandard previous works. 15% is realistic.

Extension projects (moderate complexity): Allow 10–15% of the build cost. Extensions involve groundworks and structural openings — two of the areas most prone to unforeseen conditions.

Light refurbishment or cosmetic works: Allow 5–10% of the build cost. Lower risk but still possible to encounter unforeseen conditions (particularly in older buildings).

Rule of thumb: If you are starting a project in a Victorian London terrace, allow at least 10% contingency on a well-scoped contract, and 15% if the scope has not been fully developed.

Contingency is for genuinely unforeseen events — not for scope changes you decide to make during the project.

Legitimate contingency uses:

• Ground conditions worse than anticipated (buried foundations, soft spots, old drains)
• Structural defects discovered during demolition (corroded steels, decayed joists, failed lintels)
• Asbestos or hazardous materials requiring specialist removal
• Pre-existing damp or drainage issues not visible at the time of pricing
• Services in worse condition than expected (old wiring requiring earlier replacement, failed pipes)
• Planning conditions or building control requirements that add unanticipated costs
• Price escalation in materials during a long project

Not contingency (scope changes you chose to make):

• Upgrading kitchen specification mid-project
• Adding a rooflight not in the original design
• Extending the scope after pricing
• Changing tile, flooring, or fitting selections after order

Scope changes should be treated as variations and priced separately. Mixing scope changes with contingency confuses the budget and leaves you with no buffer against genuine unforeseen events.

In a properly structured construction contract, provisional sums are used for known but unquantifiable items — for example, "drainage diversion — provisional sum £3,000." These are items included in the estimate but needing further investigation to price definitively.

Contingency is a separate allowance for genuinely unknown risks.

When reviewing a contractor's estimate, check whether provisional sums are realistic and whether contingency has been allowed separately. A low-looking estimate with no contingency and low provisional sums will almost certainly run over budget.

A well-structured budget for a London home project looks like:

1. Build contract sum: The agreed contract price with the principal contractor
2. Provisional sums: Items within the contract not yet fully priced (e.g., drainage works subject to CCTV survey results)
3. Client-direct costs: Items outside the main contract — kitchen units, appliances, floor finishes, specialist lighting, AV systems
4. Professional fees: Architect, structural engineer, party wall surveyor, building control
5. Planning and statutory fees: Planning application fee, building regulations fee, party wall surveyor fees
6. Contingency (10–15%): Applied to the total of items 1–5 above, or at minimum to items 1–3

Contingency should be held back, not spent. It is not part of the contract sum — it sits outside the contract and is released only when genuinely needed.

At each site meeting or financial review, your project manager or contractor should report:

• Current contract sum (original sum plus any approved variations)
• Variations instructed to date
• Anticipated final account
• Risk register — remaining potential issues that could affect final account
• Contingency remaining

If the contingency is being eroded by poor site management, cost overruns, or scope creep rather than genuine unforeseen events, that is a warning sign that needs to be addressed immediately.

Price before you design: Many homeowners fall in love with a design before knowing if it is affordable. Get a ballpark cost early — before the architect has drawn the final version — so you can adjust scope before it is expensive to do so.

Separate your wish list from your must-have list: Know which elements of the project are essential and which are aspirational. If cost overruns occur, you need a clear sense of what can be descoped.

Do not commit to kitchen or bathroom specifications too early: These are often the highest-risk areas for budget overrun. Prices for kitchens in particular range from £5,000 to £100,000+ for a similar sized room. Define a budget for fittings and finishes before the project starts, not after the structure is built.

Get a fixed-price contract where possible: For well-scoped work, a fixed-price JCT contract reduces financial risk compared to a cost-plus arrangement. The contractor absorbs the risk of materials and labour costs within the agreed scope.

Review your contingency use monthly: Keep a running record of what contingency has been used and what remains. If you are at 80% of contingency with 40% of the project to go, increase your contingency before the situation becomes critical.

If the contingency is exhausted and additional costs are arising:

1. Call a project meeting immediately — understand the cause, the current final account projection, and the remaining risk
2. Agree what variations are genuinely necessary and what can wait
3. Consider whether any elements of the project can be deferred to a later phase
4. Review the scope of remaining works and identify any value engineering opportunities
5. If costs are significantly over budget due to contractor error or misrepresentation, seek legal advice

Running out of contingency is not automatically a crisis — but it must be managed proactively. Ignoring it leads to disputes, incomplete projects, and significant additional costs.

rcbGroup provides transparent cost plans and monthly financial reporting to our clients, ensuring the budget position is always clear and contingency is used only for genuine unforeseen events.