I am not sure which prolongation costs I can claim

Prolongation costs are the extra costs of being on site longer. They are often worked out as tender preliminaries divided by the contract weeks, times the overrun. That may not match how your subcontract values the cost, the period adopted, or the resources actually affected.

There are two different things in play. One is extra time on site. The other is more resources on site than you planned, often called thickening. They can fall in the same weeks, so shared payroll, plant, supervision and site costs have to be split properly. Otherwise the same money is claimed twice, and your main contractor has a reason to doubt every other line.

Each cost falls into one of three groups. A supported cost has payroll or invoices behind it and allocation sheets placing it on this job. A conditional cost has some proof, such as payroll, but its link to this period and event is not yet shown. An unsupported cost, such as an allowance with no source, is left out. Anything already paid in an agreed variation comes off.

Keep extra time on site apart from extra resources in the same period, and name the records behind each. Note shared resources and credits before any cost is assigned. Sort out any overlap before an amount goes into the claim, and say what is assumed. Work the two calculations separately, with payroll, plant and invoices behind each line.

Delay & Disruption Claim builds the claim this way if you want it done for you. We take the period from a named source and never assess it ourselves. Each line is built from the records, with every assumption stated.