1590. When an architect or builder undertakes the construction of a building or other works by contract, upon a plan and specifications, at a fixed price, he cannot claim any additional sum upon the ground of a change from the plan and specifications, or of an increase in the labour and materials, unless such change or increase is authorised in writing, or admitted by the proprietor. If in either of the above cases, the additional sum has not been fixed by agreement, it shall be determined by proof of value.