The Town and Country Planning (General Permitted Development) Order 1995

C.2.    Prior approvals

(1) The prior approval of the mineral planning authority to development permitted by Class C is not required if—

(a) the external appearance of the mine or disused mine at or adjacent to which the development is to be carried out would not be materially affected;

(b) no building, plant, machinery, structure or erection—

(i) would exceed a height of 15 metres above ground level, or

(ii) where any building, plant, machinery, structure or erection is rearranged, replaced or repaired, would exceed a height of 15 metres above ground level or the height of what was rearranged, replaced or repaired, whichever is the greater,

and

(c) the development consists of the extension, alteration or replacement of an existing building, within the limits set out in paragraph (3).

(2) The approval referred to in Class C shall not be refused or granted subject to conditions unless the authority are satisfied that it is expedient to do so because—

(a) the proposed development would injure the amenity of the neighbourhood and modifications could reasonably be made or conditions reasonably imposed in order to avoid or reduce that injury, or

(b) the proposed development ought to be, and could reasonably be, sited elsewhere.

(3) The limits referred to in paragraph C.2(1)(c) are—

(a) that the cubic content of the building as extended, altered or replaced does not exceed that of the existing building by more than 25%, and

(b) that the floor space of the building as extended, altered or replaced does not exceed that of the existing building by more than 1,000 square metres.