Build a renovation budget including a 10-20% contingency buffer.
A renovation budget has two layers. Hard costs are what you can quote: materials, trades, permits and design fees. The contingency is what you cannot, and it is not padding — it is the line that pays for the rot behind the shower, the wiring that turns out to be aluminium, and the joist that has been notched by a previous owner. Ten percent is defensible on a new-build extension where everything is known; fifteen to twenty is realistic on any older property, and once you are opening walls in a pre-1970s house, twenty is not pessimistic. Dividing the total by the project duration gives the monthly cash requirement, which is the figure that determines whether the project stalls halfway.
Home Renovation Budget
Hard costs = materials + labour + permits; contingency = hard costs x contingency%; total = hard costs + contingency; monthly cash = total / project months
10% for new construction where nothing is hidden, 15% for a straightforward remodel, and 20% or more for a pre-1970s house or anything involving structure, drainage or wiring you cannot see.
Skip hire and disposal, temporary accommodation or eating out, storage for furniture, and the finishing items — handles, trim, paint, light fittings — which together often add 10% to a job that felt fully costed.