How to Avoid an Extension Going Over Budget

Decide the design fully before pricing it

A quote against an incomplete design is a quote against assumptions, and every assumption that turns out wrong becomes a variation at a price you are no longer in a position to negotiate.

Fix the layout, the window and door schedule, the structural openings and the drainage strategy before quotes go out. If a decision is genuinely undecided, price both options rather than leaving it open.

Investigate the ground before you rely on the number

Foundations are the largest single unknown. A trial hole, and honesty about trees, drains and previous structures on the plot, converts the biggest variable in the project into something close to a known quantity.

It costs little compared with the depth of a foundation nobody planned for.

Stop changing your mind after first fix

Variations cost more than the same work would have cost if it had been in the original scope, because completed work is undone to make room for them. The cost of a change rises steeply with each stage that has passed.

At drawing stage: the cost of an amended drawing · Before first fix: usually modest · After first fix: opening up already paid for work · After plastering: expensive and visible in the finish · After completion: effectively a second small project

Order the long lead items early

Windows, doors, structural glazing, staircases and stone worktops all have lead times measured in weeks. A trade standing idle waiting for a delivery costs money and pushes every following trade back.

Put lead times on the programme and place orders against them rather than against when they are needed on site.

Keep the cost visible while it can still be steered

Ask for a running record of variations with prices agreed as they arise, not a reconciliation at the end. Approve each one in writing before it is carried out.

That single habit does more to prevent an unpleasant final account than anything else, because it turns cost into a series of small decisions you make rather than one large number you receive.

Hold the contingency, and hold it properly

A contingency is for the unforeseen, not for upgrades. The temptation, once the shell is up and looking good, is to spend it on better taps and a bigger rooflight while the foundations have yet to be excavated.

Release it as risk passes: after the ground is open and the foundations are in, most of the genuine uncertainty has gone and what remains can safely be spent on the finish.

Frequently asked questions

What is the single biggest cause of overspend?

Changes made after work has started, closely followed by foundations that go deeper than assumed. Both are addressed before anyone is on site.

How do I keep control of variations?

Insist that every variation is priced and agreed in writing before it is carried out, and keep a running total. Verbal agreements on site are where final accounts go wrong.

Should I hold money back at the end?

A final payment held until snagging is complete and certificates are handed over is normal practice and protects both parties from an unfinished ending.

Can I save money by managing it myself?

Sometimes, if you have the time and the sequencing knowledge. Trades left waiting on each other is the usual cost of learning that on the job.