Why Every London Renovation Needs a Contingency — and How Much to Hold

The most consistent financial mistake on London renovation projects is insufficient contingency. Clients who budget to the last pound for the contracted scope — and have nothing left for the unexpected — face a choice when problems arise: compromise the quality of the finish, borrow additional funds, or stop the programme mid-project. All three are worse than setting aside a realistic contingency at the outset.

This guide explains where cost overruns come from, what a realistic contingency looks like by project type, and how to manage it.

Where overruns come from

Hidden structural problems: Victorian and Edwardian London buildings were built without detailed documentation. Once walls are opened, floors lifted, or excavation begins, conditions that were invisible at survey become real: failed lintels, rot in wall plates, undersized joists, cracked party walls, inadequate foundations. These are not rare — on any whole-house renovation, some structural discovery is almost expected.

Services in worse condition than expected: Old gas pipework, lead water pipes, aluminium wiring, overloaded consumer units, failed drains. A survey identifies obvious defects but rarely shows the full condition of services hidden in walls and floors. Once work begins, the true condition becomes apparent.

Drainage: CCTV surveys before work begins are advisable but not always done. Collapsed clay drainage runs, misconnected surface water and foul drains, and proximity of drainage to new foundations are all common sources of unexpected cost in London terrace projects.

Design changes: The most controllable source of cost variation. Changes to design once work has begun — even apparently small changes, like moving a bathroom layout or adding a rooflight — create a cascade of knock-on effects through trades already completed or programmed. Change control discipline (nothing verbal, everything costed before authorised) reduces but does not eliminate variation cost.

Material and lead time changes: Specified materials discontinued, lead times extending beyond what the programme allows, substitutes requiring revised installation details. Common for specialist items: bespoke joinery, imported stone, custom sanitaryware.

Scope growth: "While you're at it" additions. The temptation when a wall is already open or a floor already lifted to extend the scope incrementally is understandable but must be costed each time. Scope growth is one of the most common drivers of budget overrun because each individual addition seems small.

What is a realistic contingency?

Contingency varies with the type and age of the building, the degree of structural intervention, and the quality of pre-work surveys:

These are not pessimistic figures — they reflect what experienced quantity surveyors and project managers see on London renovation projects consistently. A 15% contingency on a £400,000 renovation is £60,000. If it is not spent, it is returned to the client. If it is not held, a £60,000 overrun creates a crisis.

The employer's risk — what the contract does not cover

A standard JCT Minor Works contract places most unforeseen physical risk with the employer (the client). If the contractor opens a wall and finds conditions different from what was described or reasonably anticipated, the cost of dealing with those conditions is typically a variation — paid by the client. Understanding this is important: the contract sum is not a guarantee of final cost.

The risk can be partly reduced by: - Detailed pre-tender investigation (structural surveys, drain surveys, services surveys) - Good provisional sum allowances in the contract for items of uncertain scope - A clear and well-drawn specification that leaves less ambiguity for the contractor to exploit

But it cannot be eliminated. Physical conditions in Victorian buildings are inherently uncertain until the fabric is opened.

Managing the contingency

The contingency should not be disclosed to the contractor. It is a client reserve, not a negotiating position. Once a contractor knows a contingency exists, there is an incentive to find variations that consume it.

Manage the contingency formally:

• —Keep a live variation log: every change to the contract, whether instructed or proposed, is recorded with a cost estimate before it is authorised
• —Issue variation instructions in writing
• —Review remaining contingency at each monthly site meeting
• —Do not authorise new variations without understanding the cumulative position

A project manager or quantity surveyor performing the contract administration role is the most effective way to maintain contingency discipline. Without independent cost management, most clients lose track of the variation position until the final account — by which point the decisions have already been made.

Cash flow timing

Renovation projects draw down cash unevenly. Early stages — groundworks, structural steel, drainage — are cash-intensive. Fit-out stages — decoration, joinery, sanitary ware — come later. Understanding the cash flow profile helps ensure funds are available when needed.

For a 12-month project, a rough draw-down profile: 15% in months 1–2, 20% in months 3–4, 20% in months 5–6, 20% in months 7–8, 15% in months 9–10, 10% final account including retention release. This varies by project but gives a planning framework.

ASAAN provides monthly cost reports on all projects, maintaining a live variation position and contingency forecast. If you are planning a renovation and want to understand realistic budgeting for your scope, contact us for a preliminary consultation.