What Lies Beneath: Why basement developments are nothing to be scared of

Changes in planning policy may make things trickier for niche developers and private clients, but opportunities for single dwelling basement developments are still worth seizing, says Sheila McCusker…

In our article on 28th June (Ground Designs: Notes on Successful Subterranean Development), we wrote about the technology, innovations and trends relating to multi-level basements beneath larger scale developer-led schemes.

As the desire to extend single dwellings below ground continues to be popular amongst niche developers and private clients, in this article we focus on the particular challenges this type of prime residential development faces.

Challenges can include dealing with evolving planning policy, conservation area or listed building constraints, historical covenants, party wall awards, tree preservation orders and copious, often onerous, planning conditions – not to mention the numerous technical challenges which arise during the detail design and construction process. Of course the reward is a larger property allowing space for luxuries such as gymnasiums, indoor swimming pools, and spa facilities.

Keeping pace with the planners

There has been much talk in the press recently over a proposed tightening of planning law for basement developments. The Royal Borough of Kensington and Chelsea (RBKC) are currently undergoing a partial review of their core strategy and the results are to be published soon. The Council states that, “in the Royal Borough, the construction impact of basements is a significant material consideration in planning”. Concerns – primarily those of residents – centre on the impact of construction on the immediate area in terms of noise, traffic management and the continued stability of surrounding buildings.

The outcome of RBKC’s planning review is likely to be a restriction on the extent of basement excavations below gardens to no more than half the area of the garden, as well as being limited to a single storey depth – meaning that the addition of a further level below an existing basement is very unlikely to be permitted. Current policy prohibits any excavation below listed buildings. Moreover, RBKC’s core strategy review states that any basement development expected to affect trees, either on-site or on adjacent neighbouring land, must be accompanied by a full tree survey and a tree protection proposals for the construction phase – root protection zones being a key factor affecting the boundary of any new basement.

RBKC appears to be leading the way in tightening up basement planning law

Of course, not all prime and super-prime residential development takes place within RBKC, although this council appears to be leading the way in tightening up basement planning law. Other central London local authorities, particularly City of Westminster, are likely to follow suit. Our own experience of the boundary condition between RBKC and Westminster is intriguing – where planning applications for basement developments are consented on one side of the street, but firmly rejected on the other. We believe that soon it will be trickier than ever to win planning permission, regardless of local authority boundaries.

Check your deeds

Most leases on central London residential property prohibit the carrying out of works without the freeholder’s consent – the granting of which takes the form of a License to Alter. These exist to protect other leaseholders from alterations to a property that may cause damage or disturbances to adjacent properties, whether within the same building or not. Many residential freeholds in prime central London – most of which are located within the boundaries of the Great Estates such as Grosvenor and Cadogan – carry covenants that still require a license to alter, despite these properties being freeholds. Gaining the necessary permissions is becoming increasingly difficult and increasingly expensive.

Keep your neighbours on side

Similarly, the approval process for party wall awards can be lengthy and convoluted, with neighbouring owners requesting deposit amounts for any damage during the construction. This is further complicated when the use of special foundations is introduced, as these require written consent by the neighbouring owner, in addition to the party wall award.

Careful consideration should also be given to maintaining good relationships with adjoining owners who are likely to bear the brunt of construction work nuisance for the duration of the build. The potential for collateral damage to adjoining properties as a result of basement excavation needs to be carefully considered to ensure it is designed out. Appropriate insurance cover, often in the joint names of both contractor and developer, can provide important reassurance to a nervous neighbour. In the first instance a well-designed temporary works proposal, to support existing structures, is critical. This, combined with a carefully considered structural solution will be the key to allaying the worst fears of adjoining owners and their party wall surveyors.

Technical challenges

Even after all possible legislative obstacles have been addressed, the nature of construction below existing foundations means challenges can literally be unearthed at any time. A well organised contractor with the capacity to properly programme and manage the construction process is invaluable. Indeed a construction site management plan that seeks to identify and minimises risks and inconveniences associated with building in confined and difficult circumstances, is usually listed as a pre-commencement planning condition.

The most difficult and unpredictable of all technical challenges is the control of ground water penetration

As to the construction of the basement structure itself, underpinning, piling, special foundations, retaining structures are all recurrent themes. Arguably the most difficult and unpredictable of all technical challenges is the control of ground water penetration, particularly during construction. The methods for dealing with this problem are as numerous and variable as the ground conditions from which the water seeps. Whether to use contiguous piling, secant piling, sheet piling, permeation grouting, waterproof concretes, structural waterproofing, tanking or cavity drainage systems, requires specialist knowledge and joined up thinking on the part of the principle design team members. Each method has pros and cons, in terms of its suitability, cost and construction programming, as well as impacting on the potential sales area of the newly excavated floor.

In our experience, keeping ground water at bay is an area where we tend to favour a belt and braces approach. It may be a more costly solution to incorporate a backup waterproofing system in the design, but it will be much less expensive than dealing with a constructional failure post completion.

Challenges versus rewards

Given the various obstacles one might face, not to mention the challenge of increasingly active, organised and well-funded residents’ campaigns to restrict and control the number of basement developments, it isn’t surprising that putting together a comprehensive and technically competent planning submission can be a complex process. This in turn affects the cost and timescale of a project. But, with the right team behind you, opportunities for single dwelling basement developments in prime and super prime London are certainly worth seizing

In our experience, basement developments can inflate square footage of a property by anything from 30 per cent to as much as 60 per cent, depending on the configuration of the existing building.

As the value of a super-prime square foot sits comfortably above the £2,000 mark these days, such a percentage hike makes an enormous difference. And with basement developments typically housing conveniences such as gymnasiums, treatment rooms, swimming pools, home cinemas and more – all of which are becoming essential elements of the super-prime home – the “wow” factor this additional space brings is undeniable.