have been made to, and be held by, a mortgagee approved by the Secretary as responsible and able to service the mortgage properly;

involve a principal obligation (including such initial service charges, appraisal, inspection, and other fees as the Secretary shall approve) in an amount not to exceed 90 per centum of the appraised value (as of the date the mortgage is accepted for insurance) of a property, urban, suburban, or rural, upon which there is located a dwelling designed principally for residential use for not more than two families in the aggregate, which is approved for mortgage insurance prior to the beginning of construction, the construction of which is begun after September 1, 1951. The principal obligation of such mortgage shall not, however, exceed $8,100 if such dwelling is designed for a single-family residence, or $15,000 if such dwelling is designed for a two-family residence except that the Secretary may by regulation increase these amounts to not to exceed $9,000 and $16,000, respectively, in any geographical area where he finds that cost levels so require: Provided, That if the Secretary finds that it is not feasible within the aforesaid dollar amount limitations to construct dwellings containing three or four bedrooms per family unit without sacrifice of sound standards of construction, design, and livability, he may increase such dollar amount limitations by not exceeding $1,080 for each additional bedroom (as defined by the Secretary) in excess of two contained in such family unit if he finds that such unit meets sound standards of livability as a three-bedroom or a four-bedroom unit as the case may be;

have a maturity satisfactory to the Secretary but not to exceed thirty years from the date of the insurance of the mortgage;

contain complete amortization provisions satisfactory to the Secretary;

bear interest (exclusive of premium charges for insurance) at not to exceed 4½ per centum per annum on the amount of the principal obligation outstanding at any time;

provide, in a manner satisfactory to the Secretary, for the application of the mortgagor’s periodic payments (exclusive of the amount allocated to interest and to the premium charge which is required for mortgage insurance as herein provided) to amortization of the principal of the mortgage; and

contain such terms and provisions with respect to insurance, repairs, alterations, payment of taxes, default reserves, delinquency charges, foreclosure proceedings, anticipation of maturity, additional and secondary liens, and other matters as the Secretary may in his discretion prescribe.