The Town and Country Planning General Development Order 1988

Class B

Permitted development

B.  The enlargement of a dwellinghouse consisting of an addition or alteration to its roof.

Development not permitted

B.1  Development is not permitted by Class B if—

(a) any part of the dwellinghouse would as a result of the works, exceed the height of the highest part of the existing roof;

(b) any part of the dwellinghouse would, as a result of the works, extend beyond the plane of any existing roof slope which fronts any highway;

(c) it would increase the cubic content of the dwellinghouse by more than 40 cubic metres, in the case of a terrace house, or 50 cubic metres in any other case;

(d) the cubic content of the resulting building would exceed the cubic content of the original dwellinghouse—

(i) in the case of a terrace house by more than 50 cubic metres or 10%, whichever is the greater,

(ii) in any other case, by more than 70 metres or 15%, whichever is the greater, or

(iii) in any case, by more than 115 cubic metres; or

(e) the dwellinghouse is on article 1(5) land.