This is dependent on whether the work is covered up to hide defects or whether the covered up work needs to be valued and certified.
If a contractor covers up work and there is no specific contractual clause dealing with this then the architect will be forced to make a decision regarding the opening up of work. Additional costs may be applicable in accordance with the contract. If a relevant clause is included in the contract then depending on the precise situation and wording of the clause, the contractor may be in breach of contract, and the employer may be entitled to damages.