You might think that subterranean development would not conflict with “The fundamental aim of Green Belt policy” which ”is to prevent urban sprawl by keeping land permanently open” (Paragraph 79 of the NPPF).
An inspector determining an appeal against the refusal of permission for a subterranean extension to a house in Worcestershire thought differently, however, (DCS Number 400-018-076). He noted that the construction of new buildings in the green belt should be regarded as inappropriate development, one exception being for “the extension or alteration of a building provided that it does not result in disproportionate additions over and above the size of the original building”. The building had already been extended. The inspector calculated that, when comparing the original dwelling to the one that would result if the proposal were to go ahead, the outcome would be disproportionate. He accepted that there would be no significant harm to openness but nonetheless decided that the development would ….”conflict with the fundamental aim of Green Belt policy which is to prevent urban sprawl by keeping land permanently open”.
In northwest London, on the other hand, an inspector appears to have seen things the Blog’s way. This case (DCS Number 400-017-866) involved a proposal to construct a new basement beneath the existing house to provide leisure facilities including an indoor swimming pool, sauna and steam room. The inspector acknowledged that the new basement would add considerably to the internal floor area of the house. He reasoned, however, that it would not increase the footprint of the building and would not have a material effect on the external appearance of the building or on its visual bulk in the green belt. Accordingly, he concluded that the development could be categorised as not amounting to a “disproportionate addition” to the existing building and, hence, as “not inappropriate” development in terms of green belt policies.
Information concerning house extensions in green belt areas can be found at section 12.632 of DCP Online.