Any municipality which includes an area which has been designated as a housing development zone pursuant to section 8-378 shall provide, by ordinance, for the fixing of assessments on all commercial and residential property in such zone which is improved during the period when such area is designated a housing development zone and, in the case of residential property, is occupied by families whose income is less than one hundred fifty per cent of the median family income of the municipality. Such fixed assessments shall be for a period of eleven years from the time of such improvement and the municipality shall defer any increase in assessment attributable to such improvements according to the following schedule: