Three hundred and forty pounds. That is what the extra double socket and the moved radiator came to on a kitchen refit, and none of it was on the invoice, because none of it was written down.
Variations are where the margin goes on small jobs. The client stands in the doorway, asks if you could just move that while you are there, you say yes because you are a reasonable person, and three weeks later you are arguing about a number nobody agreed.
The fix is a voice note. You step outside, press record and say what was asked for. Client wants the socket moved to the left of the hob, about a metre of cable, half an hour, plus making good. Thirty seconds.
The system turns that into a variation. It is numbered, it has the date and the job name on it, and it is priced from your rate sheet. It goes to the client as a short message with an approve button, before the work starts. If they tap approve, the variation attaches itself to the job and the final invoice picks it up. If they do not, the socket stays where it is and you have a record that you asked.
Most clients approve within the hour. They were standing there asking for it, after all. The ones who hesitate at a price are the ones who would have disputed it later anyway, and now that conversation happens on day one instead of at the invoice.
The log also does something quieter. At the end of the job you can see that the quote was 8,200, the variations were 1,140, and all eleven of them were approved. That is a different conversation to have at final account.
We build it on WhatsApp or text, whichever your clients already reply to, with the rates in your own spreadsheet. The voice note is the only new habit.