You own a ground floor or basement flat in a large house converted into flats. The house next door, still occupied as a single residence, has obtained planning permission and is excavating a basement floor, or possibly two or three basement floors, to substantially increase the size of its accommodation. What, you might think, is preventing you from potentially doubling or even trebling the size of your flat by digging down to carve out new rooms?

A recent case explored this possibility for a tenant who owned a spacious ground floor and basement flat in a house in Highgate that had been converted into three flats. The house itself already enjoyed a place in history (and a blue plaque) as the former home of Vinayak Damodar Savarker, the Indian philosopher and poet, before it became the subject of litigation.

The description of the flat in the tenant's 999 year lease was "the Ground Floor and Basement and Cellar Flat … being on the Ground Floor and Basement and Cellar of the Building … together with the land being the garden". As you would expect, under the terms of his lease the tenant needed the landlord's consent to carry out alterations to his flat. The tenant had applied to the landlord for permission to carry out substantial development works, which included excavation, and the question the court was asked to consider in the litigation that followed the landlord's failure to give consent was whether the sub-soil beneath the basement and garden of the flat was included within the property that had been granted to the tenant in the lease.

The tenant argued that his neighbour in the second floor flat could develop the roof space between his ceiling and the underside of the roof, as it was specifically included in his flat. The tenant of the ground floor flat took this to suggest that the landlord had intended to dispose of everything, when granting the leases. The court was not persuaded by this argument as in its view, when the lease was granted in 1975, no landlord would have contemplated the possibility of a tenant wishing to extend downwards by basement excavation works as this was long before the current appetite for digging down had taken hold. Not unusually the lease required the landlord to maintain and repair the structural parts of the building, including the foundations and roof. Although some parts of the structure of the building clearly were within the individual flats, the court's view was that the foundations were "in a class of their own" and that there was "a world of difference" between developing upwards into a roof void and "digging under and into the foundations". As is also usual, the lease granted rights to the tenant for the passage of water, electricity and other utilities through pipes and wires "in or under or passing through the building …".

The judge considered that the use of the word "under" clearly suggested that the sub-soil beneath the flat was not included, as if it had been, the right in the lease to use the services passing under the building "would have been completely unnecessary".

This case emphasises the importance of understanding the precise extent of the property included in a lease and the rights and restrictions that come with it, when purchasing a flat. This is of particular importance where alterations are envisaged to appreciably increase the value of the property or simply to increase one's enjoyment of it.