7-12-4242. Application of provisions relating to supplemental revolving fund. The provisions of 7-12-4241 through 7-12-4258 shall not be applicable to any improvement unless the council shall find that 80% or more in area of the total parcels to be assessed for such improvement have been improved by the erection of permanent buildings or structures thereon having a value greater than the value of such parcels without such improvements according to the last assessment roll.