With space at a premium, particularly in urban areas, the excavation of a basement is one way to extend a house within its existing footprint. The works involved will almost certainly be 'development of land' for which planning permission is required.

In Eatherley v London Borough of Camden, the High Court was asked to consider whether such development is permitted under Class A of Part 1 to Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 or whether specific, express grant of planning permission by the local planning authority is required.

Class A of the GPDO permits 'the enlargement, improvement or other alteration of a dwellinghouse' subject to certain limitations and conditions. The GDPO is silent as to whether this includes basement extensions, but (by the same token) nothing in the GDPO says that it does not.

In the Eatherley case, the defendant council had granted a Certificate of Lawfulness confirming that the excavation of a single storey basement to a terraced house in London was lawful under GDPO. A neighbour challenged that decision.

The Court found that the excavation of a basement is capable of falling within Class A. However, it decided that there 'must be a point where the excavation, underpinning and support for a basement for a dwellinghouse becomes an activity different in character from the enlargement, improvement and alteration of that dwellinghouse.'

The Court found it necessary for the decision maker to consider whether 'as a matter of fact and degree, the single process of making the basement amounts to different activities, each of substance, so that the one is not merely ancillary to the other'.

Cranston J considered that 'in the context of an original 'two up two down' terrace house in suburban London...the development of a new basement, when there is nothing underneath at present, could well amount, as a question of fact and degree, to two activities, each of substance. There is the enlargement, improvement and alteration aspect, but there is potentially also an engineering aspect of excavating a space and supporting the house and its neighbours. That is the position, even though the latter is necessary to achieve the developer's aim, indeed is indivisible from it'

Cranston J would not be drawn on whether or not (in this particular case) the creation of a basement did amount to two separate activities. This, he decided, was a matter for the planning committee.

The council's decision to grant a Certificate of Lawfulness was nevertheless quashed on the basis that the council failed to consider whether the engineering works involved in the creation of the basement constituted a separate activity of substance. 'The council needed to address the nature of the excavation and removal of the ground and soil, and the works of structural support to create the space for the basement.'

The decision therefore leaves the principal question open to be determined as a matter of fact and degree on a case by case basis. As such, it opens the possibility to local authorities taking enforcement action against basements excavated without express planning permission which, in their view, involve substantive engineering works not permitted under the GDPO. The prudent developer, wanting to avoid such risk, would therefore have to apply for planning permission or for a certificate of lawfulness as in Eatherley. The local authority must also direct itself to consider whether any engineering works involved in the creating the basement constitute a separate activity of substance taking account of the planning implications of the works.