Homeowners and developers should still be able to build good-sized subterranean extensions, but the days of the mega-basement are dead and buried... 

Recent years have seen so-called "iceberg homes" become increasingly popular in prime London; Andrew Lloyd Webber, Nicole Kidman and Roman Abramovich are among the wealthy Londoners to have reportedly dug enormous basement extensions under their properties. 

However, councils are moving rapidly to stop the digging, and on 2 December of last year the Royal Borough of Kensington and Chelsea's (RKBC) tough new policies were approved by the Government's Planning Inspector. While larger basements are still permitted by RBKC, their scale will be strictly controlled, with restrictions on the extent to which the basements can extend under the garden, a total ban on basement developments under listed buildings, and a restriction (in most cases) to a single storey basement. Other councils are likely to follow suit; Islington launched a consultation on the issue one week after RBKC's proposals were approved.

Basement extensions are not, however, just the preserve of the super rich. More modest basements can often be constructed without the need for planning permission, although the restrictions are tight and the Building Regulations will need to be considered. Currently, turning existing cellars or basements into living spaces will usually count as permitted development under the GPDO.

Moreover, excavation works that fall under the existing footprint of the building, and involve no external alteration, are also unlikely to require planning permission - unless the building is listed.

Some larger basement extensions may also be permissible under Class A of the GPDO, which permits the enlargement and improvement of a house provided criteria governing the size and placement of the extension are met. These criteria are stricter if the property is within a Conservation Area. With that said, the government is reviewing these planning laws, so it is possible the rules may be relaxed slightly in the future.

If you dream of a six car garage, a swimming pool, gym sauna and cinema, you may need a bigger house...

However, anything more major, including additions such as light wells, will almost always require full planning permission. Excavation and enlargement works will also require building control approval, meaning that the works must meet the Building Regulations criteria. These include criteria relating to fire safety, sound, ventilation, drainage, and electrical safely, amongst other things.

Homeowners should also consider that other factors may restrict their proposed project. For example, if their property is held under a lease, it is likely that they will need permission from the freeholder before any works are undertaken. Moreover, restrictive covenants against their title may preclude certain building (or at the very least require consent). Building works may obstruct rights of way, and a basement sharing a wall with another property will require a Party Wall Award. Even the presence of a tree may mean that development may be hampered; if a Tree Preservation Order is in place, the homeowner must be careful that the development does not harm that tree.

Ultimately, it is the vast multi-storey basement complexes that make the headlines, annoy the neighbours (which, in prime London, include Jimmy Page and Joan Collins, both of whom have reportedly criticised basement works), and it is those projects which RBKC successfully sought to curtail.

Many other local authorities are now likely to develop their own proposals in light of the recent policy change. However, more modest developments should not require planning permission, and it is hoped that the government review will lead to slightly more flexibility (for example, the ability to put in light wells). Even if planning permission is required, the RBKC guidelines still allow for a generous extra portion of living space, albeit that some may need to rein in their ambitions.

The upshot is that, if you dream of a six car garage, a swimming pool, gym, sauna, and cinema, you may need to buy a bigger house. Homeowners should still be able to build good-sized basements; the days of the mega-basement, however, may be dead and buried.

This article by Edmund White originally appeared in the PrimeResi Journal in January 2015.