(a) The standards prescribed in §40.4 are applicable to residential structures designed after the effective date of this part. If the design of a structure commenced prior to that date, the standards shall be made applicable to the maximum extent practicable, as determined by the head of the department, agency, or instrumentality of the United States concerned. If no design stage is involved in the construction or alteration of a residential structure, the standards of §40.4 shall be applicable to construction or alteration for which bids are solicited after the effective date of this part.

(b) The standards prescribed in §40.4 are not applicable to:

(1) Any portion of a residential structure or its grounds which need not, because of its intended use, be made accessible to, or usable by, the public or by physically handicapped persons;

(2) The alteration of an existing residential structure to the extent that the alteration does not involve work which is related to the standards of this part; or

(3) The alteration of an existing building, or of such portions thereof, to which application of the standards is not structurally feasible.