Planning rules for basement extensions
Before we delve into the nitty gritty of planning rules for basement extensions, it’s worth checking whether your basement extension will require permission at all. If you’re unsure, you can see these articles:
Do I need planning permission for a basement extension?
Do you need planning permission for a basement conversion?
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Councils will have their own design guidance for basements as an SPD (Supplementary Planning Document). It’s really worth scrutinising this document early on, prior to the design phase.
Rules on light wells for basement extensions
Light wells are a common feature for basement extensions, allowing natural light and ventilation to the space, necessary for a habitable room like a bedroom or study.
If you’re proposing a new light well or extending an existing one, you will need planning permission. Light wells and other interventions visible from the street will be of particular interest in the planning process.
If you can, it would be preferable to locate the light well to the rear or side of the house as there will be more design flexibility in terms of appearance, railings and size. Light wells to the front will be subject to a particular set of design rules where size will need to be minimised and it may instead be necessary to opt for a skylight or grilles.
A good landscaping design can really help minimise/soften the impact of a light well.
How big can you build a basement extension?
Rules on basement sizes vary depending on your council, so again it’s important that you check their specific design guidance.
In certain London boroughs, where basement development is widespread, there are strict restrictions on the total size of the basement created. Many London boroughs are now using standard criteria, creating more policy consistency throughout the city. Common criteria for basement sizes in SPD (supplementary planning documents) are as follows:
- They can be one storey only.
- Basements can be no bigger than the size of the property footprint + 50% of the garden.
- The local planning authority is also likely to look at the characteristics of the site. If there is potential damage to existing trees or the water environment, the size may be restricted further.
What’s the maximum head height for a basement room?
Guidance on this also varies from council to council. For example:
Camden requires ‘all habitable rooms within basement accommodation to have a minimum headroom of 2.3 metres. The exception to this will be existing basements which may have a head room of 2.1 metres.’
Westminster’s SPD refers to a minimum room height of 1.9m to meet fitness standards ‘set out in the Housing Health and Safety Rating System (HHSRS) under the Housing Act 2004.’
Wandsworth’s SPD says that ‘there is no statutory minimum requirement for ceiling heights, although the staircase should provide 2.0 m headroom. It is, nevertheless, generally recommended that residential accommodation should have a minimum ceiling height of 2.15m where possible. This is covered in more detail in the Mayor’s Housing Supplementary Planning Guidance (SPG), and for stairways, in the Building Regulations.’
You should always seek advice from your local planning authority prior to the design stage. However, with complex projects it’s usually best to do this via your architect, who will know the best way to approach them and present your intentions.
It’s likely that you will need a team of design professionals to help you prepare your planning application. The first step is usually to hire an architect, who can then help you hire other necessary professionals (usually a structural engineer, and party wall surveyor) and ensure you meet the appropriate statutory obligations.
Read more about basement development from our blog:
Basement extension ideas and inspiration
How much does a basement extension cost?
Finding the right architect for your basement extension
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