Gary Hilton, of Ashcroft Avenue, Sidcup, had built a two-storey extension to his home pursuant to a planning consent granted in 2000.
However, he wished to build a further single-storey extension and applied to the London Borough of Bexley for prior approval.
He argued that the extension would be "permitted development", not requiring planning consent.
However, the council disagreed and its decision was subsequently upheld by a planning inspector.
The challenge to the inspector's ruling hinged on the meaning of the phrase, "the enlarged part of the dwellinghouse", in Class A of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development)(England) Order 2015 (the Order)
Class A grants planning permission for the "enlargement" of a dwellinghouse. Paragraph A.1(g) provides that development is not permitted where the "enlarged part" would have more than one storey, and would (i) extend more than 6 metres (or 8 metres for a detached house) from the rear wall of the "original dwellinghouse", or (ii) exceed 4 metres in height.
Together, the existing and proposed extensions would extend less than 6 metres from the rear wall of the original dwellinghouse.
However, the inspector held that the "enlarged part" was not merely the extension proposed, but also included the earlier extension. The consequence, he found, was that the proposal was not permitted development, as the pre-existing extension had more than one storey.
In upholding Hilton's challenge, Judge Robin Purchas QC held that the inspector had erred in law.
On a true interpretation of the order, the "enlarged part" of a dwellinghouse for the purposes of Class A included only that which was being proposed under Class A. The earlier extension should therefore have been left out of account.
Hilton's lawyers said outside court that the ruling would probably require amendment of the secretary of state's technical guidance on householder permitted development rights which was published in 2014.
Hilton v Secretary of State for Communities and Local Government (CO/309/2016).