Section 33. No project for low-rent housing involving the construction of new dwellings shall be undertaken by a housing authority unless the city or town has entered into an agreement with the housing authority providing that, subsequent to the initiation of the project and within five years after the completion thereof, there has been or will be elimination by demolition, condemnation, effective closing or compulsory repair or improvement of unsafe or unsanitary dwelling units situated in the locality or metropolitan area substantially equal in number to the number of newly constructed dwelling units provided by such project; provided, that where more than one family is living in an unsafe or unsanitary dwelling unit, the elimination of such unit shall count as the elimination of units equal to the number of families accommodated therein; and provided, further, that such elimination may, in the discretion of the department be deferred in any locality or metropolitan area where there is an acute shortage of decent, safe or sanitary housing available to families of low income; and provided, further, that this requirement shall not apply in the case of any low-rent housing project located in a rural nonfarm area, or to any low-rent housing project undertaken under the provisions of section thirty-four.