Where the construction of a main sewer is deemed by the Director of Public Works and the City Engineer to be necessary in any block proposed to be improved by private contract, then and in such case no work, except grading, involving the construction of a pavement on such block, shall be permitted to be done until such main sewer shall have been constructed with side sewers and other appurtenances as in this Section hereinafter provided for and regulated.
Where a main sewer has already been constructed in a block and side sewers and other appurtenances to such main sewer are deemed necessary by the said Director and City Engineer, the construction of the same shall be conditioned for in the private contract in this Article referred to.
In the case of the construction of a main sewer in any block, no permission for the construction of the same by private contract shall be granted unless such contract is signed and conditioned for the construction of such sewer for its entire serviceable length between the main street crossings, or main street intersections, as may be determined by the City Engineer, with side sewers and other expedient and essential appurtenances as may be required by the City Engineer, under such regulations as may be prescribed by him, and approved by the Director of Public Works.