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The trend for extending below ground has been on the up in the last few years. The basement boom is especially evident in London, where rising land values and the underlying pressure for space make 鈥榙ig-downs鈥� an appealing option for wealthy homeowners looking to carve out more room in costly boroughs.

But has a judicial decision just before Christmas made them less attractive? Now more modest basement-builders have been left confused by a ruling that all homeowners need formal planning consent to build underground.

The decision came in the first High Court case to consider the extent to which residential subterranean excavation can take place relying on the current regime of permitted development rights, which allow for smaller changes to a property.

A Kentish Town resident brought the judicial review against his neighbour, challenging Camden Council鈥檚 decision allowing them to extend with relative ease under these rights, rather than going through the formal planning process.

鈥淚t is a matter of controversy in the planning world and there is a split between local planning authorities as to the correct answer,鈥� Mr Justice Cranston noted.

The ruling was built on the argument that excavation constitutes an 鈥渆ngineering operation鈥� and as such is not included within permitted development rights. It means local authorities must now consider the engineering aspects of any extension separately from the main development plan, and thus many basement proposals are likely to require specific - and more costly - planning permission.

In its defence, Camden Council countered that 鈥淐entral government regulations on basement permitted development rights are not drafted clearly and are open to different interpretations.鈥� So did things just get clearer for everyone? Or will people be put off by the thought of rummaging through reams of legal lexicon?

Homeowners can find themselves buried in paperwork as it is. In response to resident concerns about the impact new basements may have on neighbouring properties and the natural environment - including noise, groundwater issues and (in some cases) even subsidence - many London boroughs have already introduced strict guidelines for so-called 鈥渂asement impact assessments鈥�.

[The ruling] means local authorities must now consider the engineering aspects of any extension separately from the main development plan

Now more than ever, homeowners need to make sure they have done everything they can before making any planning application. But how?

First, they should obtain their own detailed study beforehand, incorporating all surveys required by the London boroughs to gain consent. Depending on what borough the property is in these could include site investigation works for soil investigations; ground movement & hydrogeology surveys; construction management surveys; and flood risk assessments.

Second, they should obtain a structural documents to assist their application. This should include a detailed CAD scheme design for the structure; structural scheme calculations; and a structural planning report that will include assumed method statements and a description of the proposed structural solutions.

Third, once planning is granted, it is vital to enlist expert structural design services for any type of basement, be that one under an existing house; an extension to an existing basement; basements under gardens; or sizeable basements for commercial or large residential units.

These should accommodate for the technical issues that arise from different ground conditions and site topographies, to inform the best advice on the most suitable structural solution for any site. And of course, with the ever changing requirements for basement impact assessments, it is vital to enlist a structural engineer who constantly reviews the dynamic planning environment, too.

If they do, homeowners can still extend below, above board, without finding themselves stuck between a rock and a hard place.

Chris Atkins is the managing director of London based structural engineering consultancy Symmetrys