Notwithstanding any other provisions of this chapter, in connection with any mortgage insured pursuant to any section of this chapter which covers a property upon which there is located a dwelling designed principally for residential use for not more than four families in the aggregate, the Secretary is authorized, upon such terms and conditions as he may prescribe, to insure under said section the amount of any advance for the improvement or repair of such property made to the mortgagor pursuant to an "open-end" provision in the mortgage, and to add the amount of such advance to the original principal obligation in determining the value of the mortgage for the purpose of computing the amounts of debentures and certificate of claim to which the mortgagee may be entitled: Provided, That the Secretary may require the payment of such charges, including charges in lieu of insurance premiums, as he may consider appropriate for the insurance of such "open-end" advances: Provided, further, That only advances for such improvements or repairs as substantially protect or improve the basic livability or utility of the property involved shall be eligible for insurance under this section; Provided further, That no such advance shall be insured under this section if the amount thereof plus the amount of the unpaid balance of the original principal obligation of the mortgage would exceed the amount of such original principal obligation unless the mortgagor certifies that the proceeds of such advance will be used to finance the construction of additional rooms or other enclosed space as a part of the dwelling: And provided further, That the insurance of "open-end" advances shall not be taken into account in determining the aggregate amount of principal obligations of mortgages which may be insured under this chapter.