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Are basement excavations worth it for clients?

A buyer likes a property but it has no potential for extension or roof conversion and he needs more space, so, says Jeremy Leaf FRICS, what’s the solution?

Jeremy Leaf

Basement creation image basement excavationsDigging deep to create a basement may be the answer for some of your clients but these underground projects also mean digging deep into your pockets.

Basements are expensive to create and will only be worthwhile if you live in a high value area where the price per square foot of high quality living space is substantially higher than the cost of construction.

The new basement is likely to have a restricted view onto a light well, rely on borrowed light or may be completely internal. If you are craving a media room, home gym, wine cellar or store that could be great. If you need extra living or bedroom accommodation that might need some careful planning and a few compromises.

Basement creation is all the rage, but beware, they need specialist architects, surveyors and builders!

Jeremy Leaf image

Jeremy Leaf

However, a well-excavated and designed basement with ample natural light and ventilation can add 10 to 15 per cent to a property’s value so prove more cost-effective than moving in many parts of London for instance. Basements first became financially viable in prime Central London but as house prices and moving costs have escalated they can now be found in wealthy suburbs too. They have proved so popular in the Royal London Borough of Kensington & Chelsea that the local authority has had to introduce specific restrictions to control the number of applications.


Before embarking on a project you need to establish whether a basement conversion will increase marketability and value sufficiently to justify the massive upheaval involved. Basement conversion is also notorious for causing disputes with neighbours. You may end up with the house of your dreams at the expense of unintentionally being perceived as the neighbour from hell!

The creation of a new habitable basement will generally require Building Regulation approval regardless of whether it involves change of use of an existing cellar, or the creation of a new larger area through excavation. The regulations are statutory minimum construction standards that ensure properties are safe, hygienic and energy efficient.


Town Planning consent will be required except in a small minority of cases. A useful initial exercise is to walk around your local area to identify those houses which have had basement constructions and then to look at related documents on the Town Planning website. This will give you a good idea of what is possible and whether it accords with your requirements. A look at www.planningportal.gov.uk is also likely to be very helpful with regard to making a planning application. In some cases, a basement can be constructed using permitted development rights but specialist advice is essential.

The High Court recently ruled that a basement development in the London Borough of Camden was not authorised under permitted development rights where engineering operations prior to building, such as excavation and structural support, amounted to ‘separate activity of substance’.

The Court quashed Camden Council’s grant of the development certification on the grounds that the Council had misdirected itself in believing the engineering operations automatically fell within the scope of so-called Class A permitted development rights.

Also, the Court further found that the Council had failed to consider the nature of the excavation, removal of the ground and soil as well as structural support required. The ruling has provided some much-needed clarity for local authorities. As a result, many more basement schemes are likely to require planning consent but local authorities will have to ensure their response is consistent with the outcome of this case.

Planning policy on basements varies but it is often very difficult for a local authority to find reasonable grounds for refusal, especially if the work does not significantly alter the building’s appearance. However, recent high-profile cases of underground extensions of several storeys, creating so-called ‘iceberg’ basements, has prompted a review of planning policy on basements.

The structural stability of a house may actually be improved by basement conversion as many Victorian homes lacked adequate foundations which can be subsequently provided by new deep foundations for the basement.

The majority of basement conversions are carried out under terraced houses and this will involve service of Party Wall notices under the Party Wall etc. Act 1996. Building a basement will involve employing specialist architects, surveyors or builders who have solid experience of this type of development – so be warned!

Jeremy Leaf BSc FRICS, independent North London Estate Agent and former RICS Residential Chairman.

For more about planning permissions, here is a useful article.

August 24, 2017

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