Creating an initial budget

Understanding build costs

The build cost is the part most people picture first — the labour and materials to actually construct your extension, conversion or improvement. It's usually the largest single line in your budget, but it's far from the only one.

Build costs are often talked about as a rate per square metre, which is a handy starting point. Just remember that the figure depends heavily on your specification — a simple, well-finished room costs very differently to one with large glazed doors, underfloor heating and a high-end kitchen.

Costs beyond the build

This is where budgets most often come unstuck. Around the build itself sits a ring of smaller costs that are easy to overlook — but together they can add up to a meaningful share of your total. Here's what to factor in.

Creating a realistic budget range

Rather than fixing on one number, set a range — a comfortable lower figure and an upper limit you won't cross. A range reflects the reality that prices move, specifications change, and quotes differ. It also gives you room to make confident decisions without feeling you've blown the budget at the first hurdle.

Start by adding up the build, the costs beyond the build, and your finishes. Then layer a contingency on top. The total gives you your upper limit; trimming the nice-to-haves gives you the lower end of your range.

Why quotes can vary significantly

It's completely normal to receive quotes that differ by thousands of pounds for what looks like the same job. A higher quote isn't automatically overpriced, and the cheapest isn't automatically a bargain. The differences usually come down to:

• What's included — one quote may cover finishes and making good, while another stops at the structure.
• Quality of materials and the level of finish being priced.
• How busy the builder is, and whether they're pricing to win the work or to fit it in.
• Assumptions about the unknowns — a careful builder may price in for things a cheaper quote ignores.

Building a contingency fund

Almost every project uncovers something unexpected once work begins — old foundations, hidden damp, a drain in the wrong place. A contingency fund of 10–15% of your build cost turns these moments from a crisis into a manageable detour.

Treat it as untouchable money set aside for genuine surprises — not a pot for upgrading your kitchen mid-project. If you reach the end and haven't needed it, that's a welcome bonus rather than a missed opportunity.

Common budgeting mistakes

A little awareness goes a long way. These are the slip-ups we see most often — and they're all easy to avoid:

• Budgeting only for the build and forgetting the fees, surveys and finishes that surround it.
• Pinning everything on a single figure instead of a sensible range with room to flex.
• Choosing the cheapest quote without checking what it leaves out.
• Skipping a contingency, so the first surprise becomes a stressful scramble for funds.
• Leaving finishes and furniture until last, then having nothing left in the pot for them.

Example budget breakdown

To show how it fits together, here's an illustrative breakdown for a typical single-storey rear extension. These figures are examples only — your project will differ — but they show how the pieces add up beyond the headline build cost.

What to do while you wait

Your decision is still pending, which makes this the ideal moment to get your numbers in order. Sketch out your range now, list every cost you can think of, and start gathering rough prices so the figures feel real. By the time approval lands, you'll be ready to act with confidence rather than guesswork.