(a) The contractor shall do any extra work and furnish any extra materials not provided for in his contract, which, in the opinion of the urban county engineer, may be found necessary or advisable for the proper completion of the work or the purposes thereof.
(b) All extra work and materials shall be ordered in writing by the urban county engineer and agreed to in writing by the contractor. Wherever unit prices for items included in such extra work are set forth in his contract, the contractor shall accept the unit price bid, as full compensation for such work; and for such items as are not covered by unit prices, he will accept, as full compensation, the actual cost of such material and necessary labor, plus fifteen (15) percent for superintendence, use of tools and plant and overhead expenses, unless otherwise agreed upon in writing.
(c) Upon completion of each separate job of work, the contractor shall furnish the urban county engineer with an itemized statement of all such items of cost for which there is no unit price provided in the contract.
(d) The cost of same shall be included in the cost of the improvement if properly chargeable thereto, otherwise it shall be paid by the urban county government.