Contingency reserve: how much is really enough

A reserve of 10–15% of the total budget is the basic recommendation for most renovation or finishing projects. It works well when the project is reasonably predictable — finishing work on an already checked structure, or a partial renovation in a home whose real condition is known, with no hidden or undocumented areas.

In these cases the usual surprises — small differences in quantities, minor execution adjustments, last-minute decisions — fit comfortably within that range without real pressure on the rest of the budget. The standard reserve works when the real uncertainty is small — not because surprises can't happen, but because their likely size is limited by knowing the actual state of the space.

Some situations need a considerably larger reserve, sometimes double. Old buildings with unknown hidden services — electrical or plumbing routes that were never updated, documented or were changed over time without records — are the typical example: nobody can know their real state until walls or floors are opened.

Demolition falls in the same risk category, because it can reveal structural problems invisible from outside — cracks, trapped damp, deteriorated materials. The same applies whenever there's no clear history of what's behind the existing finishes, and to projects involving structural changes — removing a wall, opening a new doorway, extending a room — where the real extent of the work can't be known until it has started.

A common mistake is treating the reserve as an optional fund, spent last if anything is left. In reality it should be a locked, integral part of the total budget, as firm as the money for materials or labour and unavailable for anything other than its purpose.

If the reserve has already gone on upgrades before a real surprise appears, the job risks stopping halfway exactly when a problem arises — when stopping costs most in both time and money. The safest way to protect it is to keep it separate, physically or at least clearly, from the working budget, so its availability doesn't tempt you into decisions that could have been made with the standard budget.

A few signals can be seen before work starts, so you can size the reserve accordingly: the age of the building, the lack of any documentation of previous plumbing, electrical or structural work, and visible signs of damp or deterioration on walls, ceilings or floors.

Recognising them before setting the final budget lets you adjust the reserve consciously and in time — very different from finding it insufficient after the first expensive surprise, in the middle of the work. An open conversation with tradespeople at quoting stage about what they've found on similar projects can give extra clues for a realistic reserve fitted to your project, not just a general percentage applied mechanically.