Hi Kye,
With the Commission, they usually advise on a reasonable solution based on what solutions you would be happy with. You can either request they complete the building (i.e. extend the completion date) and it would not be unreasonable to ask for compensation for the delay, or terminate the contract on the completion date on the grounds of their breach (i.e. rescind the contract) and ask for compensation for breach of contract this way. You can find another building in the meantime to complete the works after completion date passes and claim the costs for the new buildings from the old buildings as part of damages. The Commission may only be able to get you part of that. If you seek more, you may have to go to arbitration or court.