If it is found that the amounts hereinbefore specified as assessments and fees for costs and expenses prove to be insufficient to cover and defray all the necessary expenses, the council must estimate the deficiency and assess such deficiency pro rata upon all the lots and parcels of land in such city or town, and declare the same upon the basis set down in RCW 58.28.090, which additional amount, if any, may be paid by the claimant at the time when the certificate hereinafter [hereinbefore] mentioned, or at the time when the deed of conveyance hereinbefore [hereinafter] provided for, is issued.