The Secretary is authorized to insure under this section any mortgage meeting the requirements of subsection (b) of this section, except as modified by this subsection, which involves a principal obligation not in excess of 75 per centum of the limit on the principal obligation applicable to a one-family residence under subsection (b) of this section and not in excess of 97 per centum (or, in any case where the dwelling is not approved for mortgage insurance prior to the beginning of construction, unless the construction of the dwelling was completed more than one year prior to the application for mortgage insurance or the dwelling was approved for guaranty, insurance, or direct loan under chapter
37 of title
38 prior to the beginning of construction, 90 per centum) of the appraised value of a property located in an area where the Secretary finds it is not practicable to obtain conformity with many of the requirements essential to the insurance of mortgages on housing in built-up urban areas, upon which there is located a dwelling designed principally for a single-family residence: Provided, That the Secretary finds that the property with respect to which the mortgage is executed is an acceptable risk, giving consideration to the need for providing adequate housing for families of low and moderate income particularly in suburban and outlying areas or small communities: Provided further, That under the foregoing provisions of this subsection the Secretary is authorized to insure any mortgage issued with respect to a farm home on a plot of land two and one-half or more acres in size adjacent to an all-weather public road.