B. The enlargement of a dwellinghouse consisting of an addition or alteration to its roof.
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 forms the principal elevation of the dwellinghouse;
(c)any part of the dwellinghouse would, as a result of the works, extend beyond the plane of any existing roof slope which—
(i)is comprised in a side elevation of the existing dwellinghouse; and
(ii)is within 10.5 metres from a highway opposite the side elevation of the dwellinghouse;
(d)the cubic content of the resulting roof space would exceed the cubic content of the original roof space by more than—
(i)40 cubic metres in the case of a terrace house; or
(ii)50 cubic metres in any other case;
(e)other than in the case of a hip to gable enlargement, any part of the enlargement would be less than 20 centimetres from the eaves of the existing roof;
(f)it would consist of or include—
(i)the construction or provision of a veranda or raised platform ;
(ii)the construction or provision of a balcony which—
(aa)contains a platform of any description;
(bb)projects from the part of the exterior of the dwellinghouse to which it is affixed by more than 30 centimetres; or
(cc)if projected downwards in a vertical line to ground level, is within 10.5 metres of any boundary of the curtilage of the dwellinghouse opposite the relevant side elevation; or
(iii)the construction or provision of a roof terrace, whether or not it would incorporate associated railings, fencing or other means of enclosure; or
(g)the dwellinghouse is on article 1(5) land or within a World Heritage Site.
B.2. Development is permitted by Class B subject to the following conditions—
(a)the appearance of the materials used in each element of any exterior work must so far as practicable match the appearance of the materials used in the equivalent element of the existing dwellinghouse; and
(b)if any element of a window inserted on a roof slope or other element of a side elevation of the dwellinghouse would, if projected downwards in a vertical line to ground level, be within 10.5 metres of any boundary of the curtilage of the dwellinghouse opposite the relevant side elevation, then the window must be—
(i)obscure-glazed;
(ii)non-opening unless any part of the window which can be opened is, when measured at any point along the lowest edge of that part, at least 1.7 metres above the internal floor or stair of the dwellinghouse directly below that point; and
(iii)permanently maintained in compliance with the requirements of paragraphs B.2 (b) (i) and (ii).
B.3. For the purposes of Class B “resulting roof space” means the roof space as enlarged, taking into account any enlargement to the original roof space, whether permitted by this Class or not.
B.4. In determining the height of the highest part of the existing roof for the purposes of paragraph B.1(a), no account is to be taken of any parapet wall, firewall, chimney or other roof furniture or similar structure projecting from that roof.
B.5. For the purposes of paragraph B.1(b), a hip to gable extension which would result in the enlargement of the area of an existing roof slope forming the principal elevation of the dwellinghouse is not to be taken to be development extending beyond the plane of that roof slope.
B.6. For the purposes of paragraph B.1(e)—
(a)measurement to the eaves is to be made to the point where the external walls of the dwellinghouse would, if projected upwards, meet the lowest point of the upper surface of the roof; but
(b)no account is to be taken of any parapet wall or any part of the roof surface which overhangs the external walls of the dwellinghouse.