How to budget a renovation, from first principles
In the 2026 Houzz and Home UK study, the largest published survey of UK renovation activity, 38 per cent of homeowners exceeded their original budget and 3 per cent came in under. That is not 38 per cent of people being bad at arithmetic. Budgets fail on scope: the costs nobody wrote down, not the costs written down wrongly. So the method below spends most of its effort on completeness, and only then on numbers.
First, write the whole scope
Before pricing anything, list every piece of work the project involves, including the unglamorous lines that sink budgets: scaffold, skips and waste, temporary works, the drain that needs diverting, structural steel, Building Control fees, a planning application if one is needed, making good the rooms the work passes through, and redecorating at the end. On a typical extension the forgotten lines are rarely the walls and the roof; they are the ones that only exist because of the walls and the roof.
A useful test of completeness: walk the job in time order, from the first delivery to the final coat of paint, and write down everything that has to happen for the next thing to happen. The gaps announce themselves.
Price in ranges, not single numbers
A single number is a guess wearing a suit. Every line should carry a best case and a worst case, because at planning stage that spread is the truthful description of what you know. Published UK benchmarks disagree with each other substantially for the same job, mostly because of scope boundaries, VAT treatment and whether professional fees are included, so treat any single published figure as one point inside a range rather than an answer.
The width of the range is information. A line whose worst case is triple its best case is telling you where the project's risk lives, and where a real quote is worth getting early.
Know what your figures include
Three questions to ask of every number, your own included. Does it include VAT? Renovation and extension work is generally standard-rated at 20 per cent, and a quote that is silent about VAT is a fifth bigger than it looks. Does it include professional fees? Architect, structural engineer and Building Control commonly add around a tenth to a project and are missing from many published figures. And what scope does it assume? A shell-only rate and a fully-finished rate for the same extension differ by hundreds of pounds per square metre.
Contingency is a separate line, held apart
Hold roughly 10 to 15 per cent of the build cost as contingency, which is the range most 2026 UK guides recommend for substantial projects, and keep it out of the working budget so it cannot be quietly absorbed. Contingency exists for discovery: what the opened-up floor reveals, not for upgrading the taps. The fuller version of when to hold and release it is in the contingency guide.
Time the money as well as counting it
A budget that is affordable in total can still fail in March. Deposits for windows and kitchens land months before the items do; a planning decision adds a statutory eight-week wait in which little is spent and nothing progresses; the expensive trades cluster in the middle of the build. Lay the spend against the calendar and find the worst month before it finds you.
Let quotes replace estimates, one line at a time
The estimate range exists to be replaced. As real quotes arrive, the budget firms up line by line, and the difference between an accepted quote and your range is feedback about the lines still unpriced. Comparing quotes fairly is its own discipline, covered in how to compare builders' quotes.
Then track it, or the work was decorative
A budget built this way and never touched again is a souvenir. Track committed spend against forecast as decisions are made, and re-forecast when reality moves, so the overrun that is coming announces itself in month two rather than at the end. This is the part software is for, ours or anyone's: the method above works on paper, but the tracking is where paper gives up.
Common questions
How accurate can a pre-quote budget actually be?
On a well-scoped project the total tends to land inside a range of roughly plus or minus 15 per cent, and single lines can still be wildly out. The purpose of the early budget is not precision, it is completeness and an early warning system.
Should I budget from published cost guides?
As a starting point, yes, with their limitations understood: published UK figures for the same job disagree substantially depending on scope, VAT and fees, and few state their assumptions. Use them for ranges, never as quotations.
What is the single most common budgeting mistake?
Treating the first builder's estimate as the budget. It anchors every later decision, it usually describes a narrower scope than you think, and it was produced in ten minutes to win a job.