Your answer is C, unless you choose to use BY AGREEMENT bill of quantity rates. Otherwise like any compensation event it would be an add and omit based on forecast Defined Cost now.
If you can prove that the shed would have been £9k to build (including reasonable risk + fee) and the garage will be £20k (including risk and fee) then the CE sum will be for the difference. That difference then needs to be reflected in the B of Q - which the contracts is not too prescriptive on how that is done.
If you can prove that the shed would have been £9k to build (including reasonable risk + fee) and the garage will be £20k (including risk and fee) then the CE sum will be for the difference. That difference then needs to be reflected in the B of Q - which the contracts is not too prescriptive on how that is done.