Provides that in cities with a population of one million or more, a tenant or group of tenants, after proper notice to the landlord of the existence of a hazardous violation of housing codes and certification of the existence of such violation by the local agency charged with enforcement of housing codes, may contract for the repair of such condition and offset the price of such repair from his rental payments if the landlord fails to commence work to correct the condition within a certain period after notice is received.