Basement blueprint

Basement extensions: what you need to know before you call in the diggers

Basement extensions have become the norm in London as an answer to the capital’s building space shortage – often involving excavation under the gardens to the rear (and occasionally front) of the property. But an increasing number of householders outside the M25 are also converting or creating basements to increase living space, accommodating anything from additional reception rooms to underground swimming pools. 

If filling in the side return or converting the loft isn’t possible, and you don't want to sacrifice part of the garden, digging out the basement might be the only option for extending a property. But before you call in the diggers, it's essential to consider the following points: 


The planning system can be complicated and is ever-changing. As a general rule, planning permission would not normally be required for the conversion of existing cellars or basements. When it comes to creating a new basement, permitted development rights would have been relied upon in the past. However, a recent High Court decision has cast doubt on the validity of the permitted development route. Many local authorities are also using 'Article 4 Directions' to remove permitted development rights. And if the building is listed, consent will always be required. I would urge anyone embarking on this type of project to consider a certificate of lawfulness or planning permission. Councils are tightening up on requirements for planning applications often requiring Construction Method Statements, Noise Dust Vibration Reports, and Traffic Management Plans as part of the submission.


It’s very important to establish a realistic cost for any basement works from the outset. This will very much depend on your brief, as costs for a basement extension can vary enormously. For example, you could convert an existing space into a wine cellar or utility room for less than £50,000, while more extensive designs could cost £500,000 and upwards.

Site access

How easy will it be for a mechanical digger to gain access to the proposed space or will all machine traffic need to be through your principal residence? Excavations can be hand dug but this is clearly a more costly route.

Party walls

How close are your immediate neighbours? If in doubt, ensure you consult with a qualified party wall consultant.


You can’t cut corners when it comes to ensuring waterproofing. Make sure to specify a top-quality tried-and-tested tanking solution.

Flood risk

Are you within a Flood Risk Zone? If so, it doesn’t mean a basement extension can’t be considered, but you will need to take further steps to avoid any potential for flooding.

Ground conditions

The soil your property is built upon – whether it's chalk or clay, for example – will have a direct effect on the possible basement design.


Good design will usually resolve any issues over lack of light, but it is a very important factor to consider. If you are creating a storage area or utility room,  issues of light will be less of a concern against say a kitchen, bedroom or living space.


Further information

Contact Savills Planning & Development or Savills Rural

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