If there is forecast, then there is risk and under the ECC and ECSC, the (Sub)contractor is allowed to include for time and cost risk allowances - see clause 63.6 of the ECC.
The question is whether the 15% risk allowance is reasonable. I don't like standard percentages. I prefer :
- a discussion about the nature of the works being undertaken, critical dependencies etc. and, without over-sciencifying it as the risk management profession tend to do :
- identify the most likely risks (say top 5),
- what you (plural : Employer, Contractor, Subcontractor) can and will do to mitigate,
- price up the actions C / SubC will do as Defined Cost, and
- then sum of guess'estimates of residual probability x impact = £risk allowance.