Answered: NEC ECC: Add and omit under an option E Contract

I am not sure if I fully understand the question, in part because what I think you are asking is irrelevant under an option E, cost reimbursable contract. This is because you are paid Defined Cost plus Fee, so in simple terms £20k additional Defined Cost + Fee work minus £10k Defined Cost + Fee = +£10k Defined Cost + Fee that you are paid.

Note that I emphasise 'Defined Cost + Fee' which is the basis for how you are paid ... not on forecasts !