Tracking Your Build Costs

Most extension projects overrun their original budget. That overrun doesn't come from bad builders or unreasonable prices. It comes from homeowners who set a budget at the start, then don't look at it again until they're running out of money.

The budget you built during the planning stage is a prediction. The build itself generates real numbers. If you don't capture those numbers as they arrive, compare them against your predictions, and adjust your forecast, you're flying blind. By the time you notice a problem, it's too late to fix cheaply.

This isn't about being an accountant. It's about having a system that takes five minutes after each payment and tells you exactly where you stand. Set it up during pre-construction, before the first invoice arrives, not halfway through when the numbers are already a mess.

What You Need to See: Three Numbers, Not One

Most homeowners track one number: how much they've spent. That's not enough.

Professional project managers track three distinct figures for every cost category, and you should too:

Actual costs are what you've physically paid. Cheques cleared, bank transfers sent, cash handed over. This is the number most people focus on. It tells you where you've been, not where you're going.

Committed costs are what you've formally approved but haven't yet paid for. You've signed off on the electrician's quote. You've told the plasterer to go ahead. The kitchen units are on order. That money is spoken for, even though it hasn't left your account yet. Ignoring committed costs is how people convince themselves they're on budget when they're already over.

Remaining forecast is what's left from the original estimate, minus actuals, minus committed. This is the number that actually matters. It tells you how much breathing room you have for the rest of the build. If your remaining forecast is shrinking faster than work is progressing, you have a problem.

Setting Up Your Cost Tracker

You don't need project management software. A spreadsheet works. What matters is the structure, not the tool.

The Columns

Your tracker needs eight columns, with a row for each individual line item:

1. Category (matching the budget categories you set during planning)
2. Item (the specific line within the category. Record each quote, invoice, and order on its own row rather than lumping a whole category into a single figure)
3. Estimate (your original budget figure)
4. Quoted price (the actual quote you received from the trade or supplier)
5. Approved variations (any additional scope formally approved after the original quote)
6. Actual cost to date (what you've paid so far)
7. Committed (approved but not yet invoiced)
8. Remaining forecast (estimate plus variations, minus actual, minus committed)

A notes column is useful too. "Builder said this might increase" isn't a formal variation, but it's a signal worth recording.

The Categories

Match your budget categories from the budgeting and contingency leaf. Don't reinvent them. Consistency between your budget and your tracker is what makes variance analysis possible.

For a typical extension, your rows look something like this: professional fees (architect, structural engineer, building control), groundwork, structure, roof, windows and doors, first fix electrics, first fix plumbing, insulation, plastering and screeding, second fix electrics, second fix plumbing, kitchen units and worktops, kitchen fitting, flooring, tiling, decoration, external works, and contingency reserve.

That's 18 rows. Not 80. Enough to spot problems, not so many that updating the spreadsheet feels like a second job.

Click to enlarge

Spreadsheet

Extension Build Cost Tracker

Pre-configured spreadsheet with the eight-column structure (including an Item column), standard extension categories, and a worked-example tab filled in for a ~30m² extension. Works in Excel, Google Sheets, and Numbers.