The contractor who undertakes to build a structure or any other work for a stipulated price, in conformity with plans and specifications agreed upon with the land-owner, can neither withdraw from the contract nor demand an increase in the price on account of the higher cost of labor or materials, save when there has been a change in the plans and specifications, provided:
(1) Such change has been authorized by the proprietor in writing; and
(2) The additional price to be paid to the contractor has been determined in writing by both parties. (1593a)