Guide

Variations: why a fixed price changes

Almost every build has variations. The question is not whether you will have them, it is whether you find out before the work is done or after.

Last updated 13 September 2026 · Written by the Better Builds team

A variation is a change to the contracted scope of work. It can come from you, from the builder, from the certifier or from the ground. A fixed-price contract fixes the price of the work described in it; a variation changes the work described in it, and therefore the price.

Where they come from

You change your mind. The most controllable source, and the most common. Changes after the plans are approved cost far more than changes before, because approval, engineering and ordering all have to be revisited.

The site is not what was assumed. Rock, fill, groundwater, a service running through the slab footprint. This is what a provisional sum is for, but it can also generate a variation.

An authority requires something. A condition of approval, a certifier requirement, a service authority insisting on a different connection point.

Something was missed. An error or omission in the original scope. Whether that is chargeable depends on your contract and on what caused it.

What a variation should say

In writing, before the work happens: what is changing, what it costs, and what it does to the build programme. That last one is routinely left off and it matters — a small variation that delays a trade can cost more in time than in money.

How to keep control of them

Finish your decisions before you sign. Selections, fixtures, layout. Every decision you defer becomes a variation later.

Ask for the cost before approval, not after. Verbal approval on site is how disputes start. If it is not written down with a number, it is not agreed.

Keep a running total. Individually small variations add up quietly. A simple list with a running figure keeps the conversation honest on both sides.

Ask how variations are priced. The contract should say whether it is a quoted amount, cost plus a stated margin, or a rate. Know which before you need to.

The reasonable version

Builders are not trying to trap you with variations. A variation is also how a builder protects themselves against doing unpaid work when you change something. The relationship works when both sides use the same written process every time, including for the small ones.