3. In the case of an extension of a building—
(a)the plans, sections, specifications and particulars referred to in items 2, 3, 4 and 5 of Rule B in relation to the extension as if the extension was the building therein referred to; and
(b)the plans and sections as required by item 5 of Rule B of the building so far as affected by the extension,
so far as necessary to enable the district council to determine whether the proposals comply with the requirements of these regulations.