An inspector dealing with an appeal against the refusal of an LDC for two incidental outbuildings in the garden of a house in west London has very neatly set out the case law on the subject which may be helpful to cut out and keep (DCS Number 400-011-972).
The council argued that the combined floorspace of the proposed outbuildings was too large to be required for purposes incidental to a dwellinghouse. The inspector explained that:
There is no statutory definition of the word “incidental”. However, case law provides authority for how this should be interpreted by decision makers. These authorities indicate that games rooms, play rooms and utility areas are capable of being a type of use that is incidental to the enjoyment of a dwellinghouse. In the leading case of Emin v SSE  it was held that it was wrong to conclude that an outbuilding could not be said to be required for a use reasonably incidental to the enjoyment of a dwellinghouse as such because it would provide more accommodation for secondary activities than the dwelling provided for primary activities. Nevertheless, the test must retain an element of objective reasonableness and should not be based on the unrestrained whim of an occupier: Wallington v SoS for Wales ; Holding v FSS ; Croydon LBC v Gladden . On the other hand, a hard objective test should not be imposed to frustrate the reasonable aspirations of a particular owner or occupier so long as they are sensibly related to the enjoyment of the dwelling. These judgments and the findings therein serve to illustrate that with each case it is a matter of fact and degree based on the particular circumstances: Peche d’or Investments v SSE .
Accordingly, the inspector concluded that each case is fact sensitive and that the right approach based on case law is to apply an element of objective reasonableness. In the case before him he found, having regard to the particular facts and circumstances, that the proposed development fell within the definition of buildings required for purposes incidental to the enjoyment of a dwellinghouse. Accordingly, it was permitted development by virtue of the rights conveyed by Class E of Part 1 of Schedule 2 of the GPDO 2015.
The following DCP chapter is relevant: 4.3445