In The total effect we reported an appeal case (DCS Number 400-017-236) in which an inspector granted permission for the replacement of an existing dwelling, garage and outbuildings in the green belt with a new larger dwelling. The inspector based his decision on Tandridge DC v SSCLG & Syrett  in which the court held that there is no reason in principle why the objectives of green belt policy cannot be met by the application of the NPPF exception allowed to replacement buildings to a group of buildings as opposed to a single building.
In (DCS Number 400-017-994), on the other hand, the inspector declined to take an outbuilding and garage into account. This case concerned the demolition of existing extensions and the erection of new extensions to a house in the green belt. The appellant contended that, for the purposes of volume calculations, the “original building” should include the main dwelling house as well as a detached outbuilding and garage.
The inspector noted the words of the judge in Tandridge which were “I do not consider that ‘building’ should be read as excluding more than one building, providing as a matter of planning judgment they can sensibly be considered together in comparison with what is proposed to replace them.” The inspector accepted that “….this is clear judicial authority and for the purposes of applying paragraph 89 of the Framework a building could, in certain instances, be considered in the plural.” However, he noted that the garage and outbuilding were located some distance from the main house and were closely related to the main road. He noticed that they were not read as an adjunct or addition to the main house and in spatial and visual terms he did not consider that the main house and outbuildings appeared as a very closely aligned group of buildings. As a matter of fact and degree, he held, the outbuildings were seen as being physically and visually separate. Consequently, he did not consider that the outbuilding and garage should be regarded as forming part of the original building (i.e. the main dwelling) for the purposes of volume calculations.
So that tells us that it all depends.
Green belt policy is discussed at section 4.251 of DCP Online.