(a) If the purpose of such demolition is to construct a new building containing housing accommodations, no certificate of eviction shall be granted under this paragraph unless such agency determines that such new building will contain at least twenty per centum more housing accommodations consisting of self-contained family units (as defined by regulations issued by such agency, with due regard for the shortage of housing accommodations suitable for family occupancy and for the purposes of this chapter in relation thereto) than are contained in the structure to be demolished; except, however, that where as a result of conditions detrimental to life or health of the tenants, violations have been placed upon the structure containing the housing accommodations by any agency of the city having jurisdiction over such matters and the cost of removing such violations would be substantially equal to or would exceed the assessed valuation of the structure, the new building shall only be required to make provision for a greater number of housing accommodations consisting of self-contained family units (as so defined by regulation) than are contained in the structure to be demolished; and
(b) The city rent agency shall, by regulation, as a condition to the granting of certificates of eviction under this paragraph, require the relocation of the tenants in other suitable accommodations, provided that the city rent agency may, by regulation, authorize the granting of such certificates as to any tenants or classes of tenants without such requirement of relocation, where such exemption will not result in hardship to such tenants or classes of tenants and will not be inconsistent with the purposes of this chapter; and