The "GPDO Part 1 (All Classes) - LDC Appeal Decisions" document has been updated to include 1 additional appeal decision relating to householder permitted development legislation, for which the conclusions are as follows:

September 2021 - Code a01334 (appeal allowed):

• This appeal decision provides an example of where it was concluded that particular works do fall within the scope of Class B, on the basis that the works do not exceed what constitutes “the enlargement of a dwellinghouse consisting of an addition or alteration to its roof”. (*)
  [Note: The works include an “L”-shaped roof extension (or similar) that covers most (or all) of the roof of the rear projection].
  [Quote: “The GPDO at Article 3, Schedule 2, Part 1, Class B, grants planning permission for the enlargement of a dwelling consisting of an addition or alteration to its roof. The reference to ‘addition or alteration’ makes it clear that to benefit from this Class, there must be a roof structure in situ which is being added to or altered; the right conferred by Class B does not extend to the demolition and reconstruction of the original roof. Whether something would amount to an addition or alteration to the roof of a dwelling is therefore a matter of fact and degree in every case. [...] The submitted drawings show most of the original outshoot roof as being removed to accommodate part of the dormer. Even so, Class B refers to the roof of a dwelling as a whole. Nothing in Class B or the Government’s Technical Guidance implies that where a dwelling has a number of sections to its roof, for example as in this case a main roof with a lower outshoot, each section has to be treated separately. As a result, whether the proposal amounts to something more than an addition or alteration has to be considered in the context of the original roof of the dwelling as a whole, not solely the outshoot roof section. I read the submitted drawings as showing that around half of the dwelling’s original roof would be unaltered by the proposal. The drawings do not suggest that there would be other changes to the front roof slope beyond two rooflights, or that where the dormer is inset from the eaves on the main roof and outshoot, the original roof structure would not be retained. The description of the development applied for refers to a conversion, not removal of the roof. There is nothing before me to indicate that it would be impractical to construct the dormer without demolishing and rebuilding the majority of the original roof. Therefore, as a matter of fact and degree, it is not unreasonable to conclude that the proposal would involve no more than an addition or alteration to the original roof. In the recent appeal referred to above around three quarters of the original roof would have been removed and remodelled, clearly distinguishing that case on the facts from those in this appeal.”].
• Where a property has a secondary roof that’s slightly lower than the main roof, then an extension on top of this secondary roof should be assessed against Class B (i.e. rather than Class A). (*)
  [Note: The secondary roof is the roof of an original two-storey rear projection on a two-storey house].
• For the purposes of the post-06/04/2014 version(s) of Part 1 of the GPDO, where a property has an original rear projection with a side-facing pitched roof, all of which is lower than the main rear roof, then an “L”-shaped roof extension (or similar) that extends from the main rear roof onto the side roof of the original rear projection is permitted development. [Note: The roof extension, for all of its (rearmost) width, extends across the original rear eaves]. (*)
  [Conclusion: The works comply with Class B].
• The phrase “the highest part of the ... roof” relates to the property as a whole (i.e. not only to the part of the property that’s being altered or enlarged). (*)
• For example, where a property has a secondary roof that’s lower than the main roof, then an extension on top of this secondary roof can be higher than the highest part of this secondary roof (i.e. so long as the extension is not higher than the highest part of the main roof). (*)
  [Note: The extension is higher than the ridge-line of the original two-storey rear projection].

Notes:

• To view the conclusions, summaries, and decision notices for any of the above appeals, please view the "GPDO Part 1 (All Classes) - LDC Appeal Decisions" document. As a member of the Planning Jungle website, you can view the decision notices for all of the appeals on the website for no extra cost.
• Any of the above conclusions marked with a "(*)" contradict other appeal decisions. The "Reference Section" within the above document indicates how many appeals have supported and contradicted each particular conclusion.