CLASS B:- The enlargement of a dwellinghouse consisting of an addition or alteration to its roof:-

To you and I this means loft conversons, dormer windows and other roof extensions to faciltate additional roof void for habitable room space.  There will be exceptions but this is what it means to most people and extension projects.

The Legislation breaks this down into several classifications that at first hand appear fairly generous but it is not until you read the restrictions, it becomes clear that there are some very definitive constraints. 

The Governments approach in their wording is to tell you with a very 'broad brush' what your are allowed to build & then want to install the 'masking tape' to define the areas where the 'broad brush' can only be applied.

We will not repeat the wording of PD legislation here as you can read it for yourself elsewhere on this web site. 

It can also apply to putting a roof on a two storey extension being built up to eaves level under Class A, or used to turn a hip roof into a gable end.