F.2 Development is permitted by Class F subject to the following conditions—
(a)any office building as extended or altered is only used as part of, or for a purpose incidental to, the use of that office building;
(b)any extension is, in the case of article 2(3) land, constructed using materials which have a similar external appearance to those used for the building being extended; and
(c)any alteration is at ground floor level only.