such corporation or such developer for the development of housing which in the determination of the commissioner contains a substantial number of dwelling units of three or more bedrooms provided (A) that the mortgage loan for such housing shall be eligible for insurance by the United States Department of Housing and Urban Development or for financing by the Connecticut Housing Finance Authority or the Farmers' Home Administration, and (B) that the commissioner, after consultation with the United States Department of Housing and Urban Development, the Connecticut Housing Finance Authority or the Farmers' Home Administration, as the case may be, shall have determined that the mortgage loan on such housing would not be insurable in the absence of such additional financial assistance; such grant-in-aid, loan, deferred loan or advance shall be in lieu of any assistance to said housing under section 8-216 and shall be equal to the additional cost of construction caused by the inclusion of such dwelling units of three or more bedrooms in such housing, but in no event shall such grant-in-aid, loan, deferred loan or advance be greater than ten per cent of the cost of construction of such housing, as determined by the United States Department of Housing and Urban Development, the Connecticut Housing Finance Authority or the Farmers' Home Administration. The commissioner may require that any assistance in the form of a loan or deferred loan be secured by a mortgage on such housing. In the case of a deferred loan, the contract shall require that payments on all or a portion of the interest are due currently but that payments on principal may be made at a later time.