As I understand it from reading section B of :
http://planningjungle.com/wp-conten...ers-Technical-Guidance-April-2014-version.pdf
Class B
This provides permitted development rights for the enlargement of a house consisting of an addition or alteration to its roof.
Couldn't be clearer, you
can do it unless one of the limits/conditions apply. So do any of the limits/conditions apply ?
B.1
(a) 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 and fronts a highway
(c) the cubic content of the resulting roof space would exceed the cubic content of the original roof space by more than ...
(d) it would consist of or include -
(i) the construction or provision of a veranda, balcony or raised platform, or
(ii) the installation, alteration or replacement of a chimney, flue or soil and vent pipe
(e) the dwellinghouse is on article 1(5) land
B.2 Development is permitted by Class B subject to the following conditions -
(a) the materials used in any exterior work shall be of a similar appearance to those used in the construction of the exterior of the existing dwellinghouse
(b) the enlargement shall be constructed so that –
(i) other than in the case of a hip-to-gable enlargement or ...
(c) any window inserted on a wall or roof slope forming a side elevation of the dwellinghouse shall be ...
And that is it<period>.
I can't help thinking that the planners want to interpret B.1(b) so that a hip-gable would fail - by interpreting "plane of the existing roof" as stopping at the edge of that roof. That would be a perverse interpretation that would never hold up.
So assuming you don't intend going higher than the existing roof, or over the volume limits (50m^3), don't intend building a balcony or veranda, don't intend putting a new soil stack or chimney up, aren't in a national park, are going to use similar materials, and don't intend having clear/opening windows - then there is no other clause to say it's not PD.
PD doesn't need any permission, not even an LDC. That's the point - you are permitted by law to do it and can just get on with it. An LDC may be useful if you intend going right up to the line rules wise and want to get any ambiguity sorted out first, but otherwise you don't need to pay the planners a penny. Of course, that latter point is probably one of the reasons they'd rather apply for permission to do what the law explicitly says you can do.