No project for low-rent housing or the clearance of a blighted area involving the construction of new dwellings shall be undertaken by a housing authority unless, subsequent to the initiation of the project, there has been or will be elimination by demolition, condemnation, and effective closing, or compulsory repair, of unsafe or unsanitary buildings situated in the area of operation substantially equal in number to the number of dwelling units provided by the project. The elimination may, upon approval by the commissioner of employment and economic development, be deferred for a period determined by the commissioner if the shortage of decent, safe, or sanitary housing available to families of low income is so acute as to force dangerous overcrowding of those families.