The extent of a property owner’s rights to dig out or extend a basement, and the extent of the rights of adjoining owners to be protected from the effects of such work, can quickly become emotive issues.

Even though the finished basement will be hidden from view, it is common for neighbours to object to proposed basement plans because of fears around the actual works – specifically, excessive noise and vibration, the typically lengthy period of building works associated with basement development, and the potential adverse effects on the structural stability of neighbours’ houses.

Owners contemplating digging down should consider the legal framework for the work they wish to carry out, and the practical requirements of maintaining good relations with adjoining owners, as well as the technical feasibility of their development.

Relevance

The amused outrage over “mega-basements” seems quietly to have died away. A few years ago, stories of oligarchs with plans for extravagant “iceberg homes” featuring pools, car stackers, gyms and walk-in humidors appeared regularly on television (Channel 4 documentary Posh Neighbours at War from 2016, for example) and in papers such as the London Evening Standard.

Now those pages are filled with other more pressing concerns, and the world moves on. But although the media focus may be directed elsewhere, that does not mean the trend – and arguably necessity – for basement development has gone away.

The same factors that drive the market for loft conversions and side-return extensions apply equally in the context of basements. Urban centres – and London in particular – are increasingly overcrowded. Available housing stock in the areas where people actually want to live is very limited. Add to that continuing fears about economic stability, with the reality of Brexit and a recession arguably overdue, and homeowners wanting more space will be reluctant to commit to buying, and paying stamp duty on, a new home.

The economic case for basement development can be compelling in terms of cost against value added, even for a relatively modest home in the right area. Experts suggest that, for construction costs of around £4,500-£6,000 per square metre, a finished basement can add value of between £7,500 and £10,000 per square metre to a London home.

For neighbours, the concerns that they might have over a proposed surface-level extension are more acute when the development goes underground. They might not be able to see the end result, but the overall design and construction period for a new basement is likely to be significantly longer and more physically disruptive than for an equivalent above-ground extension. Added to that will be the understandable fear that, if the dig goes badly wrong, the neighbour’s house might be structurally undermined and at risk of collapse.

The legal and regulatory context

Planning permission

The planning requirements governing the creation of basement living space are continually evolving to reflect current needs and concerns. However, certain types of work may be carried out without the need to apply for planning permission. These are described as “permitted development rights”, rights conferred under the Town and Country Planning (General Permitted Development) (England) Order 2015 (the 2015 Order).

Although the 2015 Order does not contain specific provisions relating to basement developments, it is possible in most cases to convert an existing residential basement into additional living space without planning permission, provided that the usage is not significantly changed, or a separate unit created, or the external appearance of the property altered – for example, by adding a light well.

Until quite recently, most local planning authorities treated basement developments generally as permitted development. This changed with the case of Eatherley v Camden London Borough Council [2016] EWHC 3108 (Admin);[2016] PLSCS 332. The neighbour claimed that the local planning authority had misdirected itself when making its decision to grant a lawful development certificate to the building owner, because the local authority had not considered in sufficient detail whether the significant engineering works required to create the new habitable basement constituted a “separate activity of substance”. The court agreed. Even though the basement extension may have been permitted development, the associated engineering works amounted to a separate activity of substance, for which planning permission was required.

While the overall approach taken by local authorities since is not necessarily more “restrictive” – reasonable plans for a basement extension in line with the guidance set out in a borough’s supplementary planning documents may be considered likely to be approved – there is a greater emphasis on the need for formal planning permission following Eatherley and a reduced scope for the creation of new basement space as permitted development. The practical effect is that for the creation of new basements involving significant works, a new light well or the creation of a new separate unit of accommodation, planning permission is likely to be required.

Building control

When creating new living space at a property, the homeowner will need to seek Building Regulations approval from the building control department of the local authority, or from an approved inspector. This is partly to provide peace of mind that the works when completed will be signed off as structurally safe; but also, any prospective purchaser of the property after creation of the basement space will require sight of a copy of the Building Regulations completion certificate granted in relation to the completed works.

The Party Wall etc. Act 1996

Homeowners contemplating a basement dig should also be aware of the requirements of the Party Wall etc. Act 1996 (the 1996 Act), which covers works, including excavation, adjacent to a building or structure of an adjoining owner. A minimum of one month’s notice must be given to neighbours in particular circumstances relating to depth and proximity of the proposed excavation:

  • if the excavation is within three metres (measured horizontally) of an adjoining owner’s building, and any part of the excavation will extend lower than the adjoining owner’s foundations; or
  • if the excavation is within six metres of an adjoining owner’s building and any part of the excavation would meet a line drawn downwards at a 45-degree angle from the base of the adjoining owner’s foundations.

In order to navigate the notice and agreement processes under the 1996 Act, a homeowner should consider taking specialist advice from a party wall surveyor, in addition to the advice they should seek more generally from their architect.

Other consents

Depending on the nature of the homeowner’s property, and the nature of their interest in the property, further consents may be required prior to any basement development:

  • consent of the local planning authority will be required in relation to any alteration or extension of a listed building that is likely to affect its special architectural or historical features;
  • consent of the freeholder if the homeowner’s interest is leasehold; and
  • consent of any relevant estate owner, for example the Grosvenor or Cadogan Estates.

Keeping neighbours on side

Just as with an aboveground extension, effective communication with neighbours should be recognised as crucial to a successful basement development. Complying with the legal and regulatory requirements is only part of the owner’s role.

The more detail that a building owner can reasonably give in its communications with neighbours, the better. This might make the difference between a successful development and one that descends into a protracted dispute.

Communication should be early and ideally delivered personally. It helps if the basement plans are reasonable and realistic, likely timings are communicated honestly, and if there can be positive messaging around the quality of the team carrying out the design and construction of the development.

Fear can often be fear of the unknown, so being willing to answer neighbours’ reasonable questions and sensibly and proactively address concerns will usually be well received.

Final considerations

Homeowners can give themselves the best chance of successfully obtaining planning consent for their development by engaging a team – in particular an architect – with experience of both planning legislation and local planning policy, along with the technical expertise to deliver the project. Owners should keep their proposals realistic and in line with local planning policy. In central London, this will typically mean:

  • single-storey basements only;
  • basements may extend to cover the footprint of the existing house, plus up to 50% only of the garden area.

The legal and regulatory frameworks, and local authority guidance, around basement developments are fluid, though, and likely to continue to develop and evolve.

A Subterranean Development Bill – intended to make provision for the presumption against the granting of planning permission in relation to basement developments where certain conditions apply – was first brought before parliament in December 2011. It did not become law, but if concerns relating to underground development become a hot topic again in the future, it (or something similar) could be revived to create a far more restrictive environment for basement works.

This article first appeared in Estates Gazette on 5 September 2019.