Renovation Contingency Calculator

The right contingency depends on two things: how old your property is and how invasive the work is. Modern post-2000 builds undergoing a cosmetic refresh only need 6 percent. A Victorian terrace getting structural work needs 20 percent. The older the building, the more unknowns are hiding behind the plaster — rotten joists, failed damp-proof courses, buried drains, or wiring that does not meet current regs. Structural work also means opening things up, which is where most surprises come from. Hold the contingency separately from your main budget and do not spend it on specification upgrades.

Contingency covers genuine unknowns discovered during the work, not scope creep. Typical uses include: replacing rotten floor joists found when lifting floorboards, rewiring a circuit that failed an inspection, rebuilding a chimney breast that was unsafely removed by a previous owner, adding extra beams when a wall turns out to be load-bearing, or treating woodworm or dry rot. It should not cover a last-minute kitchen upgrade, adding underfloor heating because you changed your mind, or paying for a delay caused by you not signing off drawings on time.

Pre-1920 London houses were built before modern standards for foundations, damp-proofing, electrics, and insulation. Walls are solid brick rather than cavity. Floors are suspended timber sitting on poorly-ventilated voids. Pipes are often lead or old iron. When a builder opens up a Victorian kitchen floor, it is normal to find things that need fixing before the new work can go down. A newer property has fewer surprises because more of it meets current standards already. That is why our contingency table scales from 6 percent on a post-2000 cosmetic refresh to 20 percent on a pre-1920 structural job.