How do you price a variation before the work is done?

Pricing a change before it is built is not guesswork, it is forecasting against known rules, and the earlier you do it the more control everyone keeps. Start from the instruction and pin down exactly what has changed, before and after, from the drawings, so you price the change rather than the whole area around it. Take the quantities from the design: a variation not yet built still has drawings, and quantities measured from them are good until the as-built proves otherwise. Value those quantities the way the contract requires: the same work at the bill or activity-schedule rate, similar work pro-rata, and only genuinely new work at a fair rate built from first principles, naming the basis for each so it cannot be quietly swapped later. Where a part genuinely cannot be rated in advance, price it as an honest forecast and label it provisional, to be confirmed on the records once the work is done, rather than pretending to a precision you do not have. Flag any time or disruption consequence as its own separate head, because it proves differently and gets lost if folded into a rate. Pricing before the work gives both sides a number to agree or challenge while the change can still be planned, resourced and sometimes avoided, instead of discovering the cost as a fait accompli. A forecast price slightly wrong, corrected on the records, beats a perfect price that arrived a year too late to change anything.

Picture a cladding subcontractor handed an instruction to change a rainscreen detail across two elevations before any of it is installed. Rather than crack on and price it at the final account, the change is priced up front: the revised areas measured off the new drawings, the panels and fixings valued at the rates already in the priced schedule, the extra edge trims that have no rate built up from the supplier quote and the labour norm, and a flagged allowance for the fortnight the redesign adds to that elevation's programme. The main contractor sees the number while the work can still be sequenced, agrees the rated elements immediately, queries the fair-rate trims, and the two settle those with the supplier quote on the table. When the work is built the provisional trim figure is confirmed against the actual delivery tickets, and the whole change closes out at a figure agreed months before rather than becoming the centrepiece of a final-account fight.