How do I keep track of spending during an extension build?

Track three numbers per cost category rather than one: what you have actually paid, what you have committed to but not yet paid, and your remaining forecast. Reconcile after every payment so you always know how much breathing room is left, not just how much you have spent.

What is the difference between committed and actual costs?

Actual costs are what has physically left your account. Committed costs are what you have formally approved but not yet paid, such as a kitchen on order or a quote you have signed off. Ignoring committed costs is how people convince themselves they are on budget when they are already over.

What is a variation order on a building project?

A variation order is the written record of a scope change: the extra work, its agreed cost, any programme impact, and both signatures. Under the Consumer Rights Act 2015 a quote is a fixed price, so your builder cannot charge more than the quote unless you have formally authorised the extra work in writing.

Can a builder charge more than the quoted price?

Not without your formal written approval. A quote is a fixed price under the Consumer Rights Act 2015, and a JCT Homeowner Contract gives verbal site instructions no effect until confirmed in writing. No written approval means no obligation to pay the extra.

What should my contingency fund actually be used for?

Contingency is insurance against genuinely unforeseen conditions such as poor ground, hidden structural issues, or a drain in the wrong place. It is not for mid-build upgrades you decided you wanted; those are choices that should come from the main budget, not the contingency reserve.

When should I review my extension budget?

Monthly is the minimum, but the natural trigger is each stage payment: review before you release money, not after. Walk the site, confirm the work is genuinely complete, and check whether any scope was discussed that needs a written variation before it appears on an invoice.

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Phase 2 · Pre-Construction · Task 11 of 13

Tracking Your Build Costs

Most extension projects overrun their original budget. Set up a cost tracker, enforce written variation approvals, and review at every milestone to stay in control.

10 min readUpdated 2026Premium

Dashboard-style infographic showing a cost tracker with three columns (estimate, committed, actual) across extension build phases, with a variance alert on one category

Prerequisites

Complete these first

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 this guide covers

01What You Need to See: Three Numbers, Not One
02Setting Up Your Cost Tracker
03The Variation Approval Process
04When to Review
05Spotting and Handling Overruns
06Protecting Your Contingency
07Updating the Forecast

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