Here is an example of the simplified and improved permitted development system (DCS Number 400-020-975). This comes from an appeal against the refusal of a lawful development certificate for extensions to a terrace house in north London, in which the inspector referred to the various elements of the proposal as A, B and C.
“The appellant has provided a detailed breakdown of the volume calculations for the resulting roofspace of the proposal corroborated by a Surveyor with an Advanced Mathematics degree. These calculations demonstrate that the wrong formulae were utilised by the Council in determining the volume of the C1 identified element (an oblique pyramid) and the volume of C2 (a triangular prism) at the time the application was determined. The Council used the formulae 1/2 bh where b equals the area of the base of the pyramid and h is its height for C1 and bh for C2. Its volume calculation for C1 was 7.095 m³ and for C2 was 5.805 m³ making a total volume, including the hip to gable and rear dormer, at the time of the determination of 41.141 m³.”
The inspector continued “The correct formulae for C1 is 1/3bh and for C2 it is 1/2 bh . These result in a volume calculation by the appellant’s expert of 4.73 m³ for C1 and 2.9025 m³ for C2. The appellant’s expert also recalculated element A to have a volume of 12.032 m³ and element B to have a volume of 18.9 m³. The appellant’s total resulting roofspace is therefore 38.5645 m³.”
The inspector concluded that the volume of the resulting roofspace of the proposal would be less than 40m³. As such, it complied with limitation (d) (i) of Class B of Part 1 of Schedule 2 of the GPDO and would be permitted development.
Further information concerning Part 1, Schedule 2 of the GPDO can be found at section 4.3421 of DCP Online.