In common law the employer agrees to pay the contractor an all inclusive “lump sum’ price for specified works… on completion of the work. In practice the parties ‘contract out of common law’ to agree an amount to be paid as the works proceeds. The contract sum may be calculated by the contractor based on drawings and a specification, or be based on a Bill of Quantities provided by the employer (compiled by the quantity surveyor). The contract sum may be varied by the issue of contract instructions by the principal agent in instruction from the employer to add/omit/alter work