How to budget for a renovation

Almost every renovation costs more than the first number in the homeowner's head. That is not bad luck; it is the normal result of pricing a drawing before anyone has opened a floor or lifted a board. Knowing where the money goes, and holding back a buffer for it, is what separates a project that finishes from one that stalls.

Fix the scope before you price it

Decide what the finished space must do, then have it drawn, then price it. A project priced from a verbal description changes once drawings appear, and every change after the price is agreed is charged at the builder's rate rather than the competitive one. If the first quote is beyond reach, cut scope rather than quality: a smaller extension built well is worth more than a large one trimmed on materials.

• Drawings first - Price from a defined design, not a description.
• Scope list - One page per room, with who supplies every item.

The contingency and why it exists

Hold back between ten and twenty percent of the build cost. The figure is higher for older properties and for anything involving a leak, a chimney or a suspended timber floor. This is not padding. It is the expected cost of things nobody can see until the floor is up: rotten joists, a missing lintel, and pipework with no fall on it.

Contingency also covers second order effects. A fortnight's delay costs more if you are paying rent elsewhere, and a failed inspection costs the time to redo work and book the inspector back. Treat the buffer as spent from day one and you finish inside the total. Spend it on upgrades in week three and dry rot found in week eight becomes a crisis.

• Ten percent - A reasonable start for a straightforward project in a modern house.
• Do not spend it - Upgrades are what eat contingency. Decide them after the messy stage.

Where the money quietly goes

The costs that surprise people are rarely the trades. They are the items sitting outside the building work: a skip, a permit to park it on the road, scaffolding that stays up for weeks, temporary accommodation, and waste disposal charged by weight. Professional fees add another layer, including a structural engineer, a designer, building control fees and a party wall agreement if you are digging near a neighbour.

Decorating and making good after the trades leave is the line most often left off a quote. Replastered walls, new skirting and a ceiling that needs painting cost money and time, and nobody priced them.

• Scaffolding - Ask early whether it is needed and who pays for it.
• Making good - Plaster, skirting, paint and flooring after the builders leave.

Structural changes need approval

Removing a wall, opening a chimney breast, adding a beam or changing a doorway in a structural wall needs a calculation from a structural engineer and an inspection by building control, who will want to see the steel before it is boxed in with plasterboard. Skipping that leaves work which fails at the survey stage when you sell, and insurers can treat uninspected structural work as grounds to refuse a claim.

Other permissions run alongside. Altering the exterior of a listed building needs consent, and doing the work without it is a criminal offence. Extensions and loft conversions may need planning permission depending on size and location, and loft rooms have to meet fire and escape rules. Start the paperwork before trades start, because a stop notice mid job is the most expensive paperwork there is.

• Structural engineer - Beam sizes and load paths. A builder cannot certify this.
• Building control - Inspects at set stages. Call them before covering work up.

Regional rates and comparing quotes

Labour varies most with location. Trades cost more in cities and commuter belts than in rural areas, and a hard to reach site can carry a travel premium. Materials are broadly similar in price nationally, but delivery charges and lead times differ, which is why a figure quoted by a friend in another region is not a benchmark for your project. Put quotes side by side and look for the same items.

Ask each builder what is excluded and how long the work will take. A fixed price protects you when a job runs over, while a day rate transfers that risk to you and only makes sense where the scope genuinely cannot be defined. A written contract with payments tied to stages is normal for anything substantial.

• Fixed price - Safer where scope is defined. Builders price in the risk.
• Stage payments - Pay as each stage completes, never a large deposit up front.

Paying for the work safely

Never pay the bulk of the money before the work exists. A normal arrangement is a modest deposit to secure a start date, then payments at agreed stages that reflect work done. Hold a small retention back until snagging is finished, since it is the only reliable way to get the last list of small faults dealt with. Pay by bank transfer rather than cash and get a receipt every time.

Check that anyone doing building work carries public liability and employer's liability insurance. Ask to see the certificate rather than accepting a promise. Confirm in writing who pays for damage to neighbouring property, and what happens if materials fail. Paying a supplier directly also cuts the risk if the builder stops trading.

• Deposit - Enough to buy materials, not enough to make you the main creditor.
• Retention - Hold a small percentage until the snag list is complete.