Begin with the true cost, not the build cost

The most common reason a renovation or extension overruns is that the budget was never a real budget to begin with. It was a build cost. The figure a builder gives you for the walls, roof and windows is only part of what you will spend, and if you treat it as the whole picture you will be dipping into savings before the plasterboard goes up.

A realistic project budget includes the construction contract, professional fees, statutory costs and a contingency. For a typical rear extension or loft conversion, expect professional fees — architect, structural engineer, party wall surveyor, building control — to add somewhere between 10% and 15% on top of the build figure. Add planning application fees, a Party Wall Award if you are excavating near a neighbour, and any service connections or upgrades to the electricity supply. Then check the VAT position carefully: most renovation work is charged at 20%, though there are reduced and zero rates for certain conversions and for work on homes that have stood empty for two years or more. Ask your accountant before you sign anything.

Set a contingency you genuinely leave alone

Contingency is not slack in the budget. It is the money that pays for the things nobody could have seen from the drawings — the rotten lintel behind the render, the drain that runs exactly where the new pad foundation needs to go, the asbestos board under the stair.

• Standard projects: allow 10% of the total build cost.
• Period or older properties: allow 15% to 20%, particularly if you are opening up walls or digging new foundations.
• Structural work or basement digs: treat anything below 15% as optimistic.

Put the contingency in a separate account and do not count it as available funds when you are choosing the kitchen. If the project finishes without using it, that is a good outcome, not a reason to upgrade the glazing. Better still, decide in advance what your absolute upper limit is — the number at which you would stop and rethink — and write it down where both you and your partner can see it.

Understand what a variation actually costs

A variation is any change to the agreed scope after work has started, and it is where budgets quietly fall apart. Moving a socket is not a variation; moving a wall after the steel has been ordered is. The problem is not only the cost of the change itself, but the disruption, the extra labour, the reordering and the delay — all of which land on your bill.

Agree a written process before the first day on site. Every change should be described, priced, and signed off by you before the work is carried out. Ask for variations to be quoted as a fixed sum rather than an estimate of time and materials, and ask the builder to confirm how many days the change adds to the programme. Keep a simple variations log — a spreadsheet with the date, a description, the agreed cost and your signature — and update it the same week. If you leave it three months, you will be arguing about invoices from memory.

Agree a payment schedule that protects you both

Do not pay large sums up front. A deposit of 5% to 10% to secure a start date is normal; anything approaching a third of the contract before materials arrive is not. Stage payments tied to completed work are the fairest arrangement for both sides.

• Break the contract into clearly defined stages — foundations, superstructure, roof, first fix, plaster, second fix, completion.
• Pay against each stage being finished and inspected, not against the calendar.
• Hold a retention of around 5% — half released at practical completion, the remainder after a rectification period of six to twelve months.
• Never pay for work you have not seen. Visit the site, take photographs, check the stage genuinely is complete.

For larger jobs, a JCT contract (the Minor Works or Home Owner contracts are both suitable for domestic projects) sets out these mechanisms clearly and gives you a proper framework if something goes wrong. It costs very little compared with the protection it offers.

Track the running total every week

Budgets slip in small increments, not one dramatic moment. Set aside twenty minutes each Friday to update your spreadsheet: what has been invoiced, what has been paid, what variations have been agreed, and how much contingency remains. Compare the running total against your original cost plan and your absolute upper limit.

Keep a forward cashflow too — the next three months of anticipated payments. It is surprisingly common for a project to be affordable overall but to run short in a particular month because a glazing deposit and a scaffolding invoice land in the same fortnight.

Control scope creep and make decisions early

The cheapest project is the one you stop redesigning. Every late decision — the tile that is discontinued, the tap you change your mind about, the second-guessing over an island — costs money in delays and rework. Front-load your choices: tiles, sanitaryware, kitchen layouts, paint colours and light fittings should all be selected before first fix begins. Where you must keep an option open, ask your builder to price both versions in writing at the outset.

Finally, be honest about your own capacity to project-manage. Running a building site is a job, and if you are doing it alongside work and family life, the temptation to accept vague answers is enormous. A good contract, a tight variations process and a contingency you refuse to raid will do more for your budget than any amount of haggling over the quote.