Basements extensions guidance

Basements extensions guidance

This page gives guidance on basement extensions, which is also available in the basement extension guidance leaflet.

Basement extensions have become an increasingly common home improvement in Wandsworth. 

Basements can require several permissions:

• Planning permission
• Building Control
• Listed building consent
• Party Wall Agreement
• Highway licence
• Skip licence
• Parking suspension
• Streetworks licence
• Freeholder consent (if you live in a flat or leasehold house)

Planning permission is usually required for basement projects, but not always. You will always need permission if you live in a flat or listed building; for a lightwell; for related works to fences and walls; to trees in conservation areas and to trees covered by Tree Preservation Orders (TPOs).

You should refer to the Housing Supplementary Planning Document which covers these matters in more detail. 

Building regulations approval will always be required.

Many properties in Wandsworth are located in flood risk areas, so basement extension projects require specialist management, construction and design techniques. The Basement Information Centre provides advice on these matters. It will assist you in choosing appropriately qualified professionals: architects, structural engineers, and waterproofing specialists and should be referred to by your chosen team. 

Construction issues

Excavation and construction of a basement needs to be carefully planned and co-ordinated to avoid problems such as noise, dust, disturbance to neighbours, removal of spoil and storage of materials. It is strongly advised that you sign up to a Considerate Construction Scheme. Talking to neighbours and avoiding the use of grab lorries, which can block the highway, and applying for the relevant skip licences and parking permit suspensions in good time can also help make the build run more smoothly.

You are strongly encouraged to prepare a Construction Method Statement (CMS), and is a requirement for listed buildings. It must be independently verified at the cost of the applicant and Wandsworth Building Control can provide this service for you. For basement projects a CMS would be expected to cover:

• Appointment of suitably qualified professional (civil or structural engineer)
• A ground and hydrological condition report dealing with groundwater flow ensuring structural stability during excavation and demolition
• Temporary propping/temporary works and construction work, minimising disturbance
• Drilling of boreholes
• Sequence of temporary works to minimise the effect on neighbours and management of water flow
• Cumulative impacts
• Professional verification of works safeguarding amenity: noise vibration and dust from construction work
• Construction traffic management plan

Design considerations

Careful consideration of design and the impact on the street scene can save you time and money - helping you decide at the outset whether a basement extension would be feasible in your property and improving the chances of obtaining planning permission. You should refer to the Housing SPD and follow up as necessary with pre-application discussions with planning officers. Multi-level basements are not appropriate in Wandsworth.

Common design issues in Wandsworth

Front gardens: you must ensure that at least 50% of the front garden remains following excavation of a 'lightwell'. As many front gardens in the borough are extremely short, to manage the impact on the character on the street scene it recommended that a minimum depth of 2m should remain in front gardens and this is normally a requirement in conservation areas.

Rear gardens: Rear gardens can be affected by a lightwell and by the construction of a basement which runs underneath the rear garden.

Lightwells: The SPD advises that excavation of a lightwell may only be acceptable in private (not shared) gardens, and that the remaining garden meets the minimum garden space standards set out in planning policy (DMPD Policy DMH7: 10 sqm for 1 and 2 bedroom dwellings and 15 sqm for 3 or more bedroom dwellings). 

Excavations under the garden: The depth of back garden you will need to retain will depend on your property and whether it is located in a conservation area. For most properties in the borough it would be good practice to retain a depth of at least a half of the back garden. Care must be taken not to damage trees and tree roots (including those in neighbouring gardens which are likely to run under your property). It is also good practice to ensure a minimum 1m depth of soil above the basement.

Conservation areas: the Conservation Area Management Plan for your area should be referred to.

Making a planning application

If you are considering building work, it is strongly advised that you take account of the Housing SPD and then discuss your proposals with neighbours before making a planning application or formal consultation on Party Wall matters. Advice on planning policy matters, making a planning application, or on the building regulations is available on the Planning pages.

Please contact us for advice including pre-application advice at planning@wandsworth.gov.uk and/or buildingcontrol@wandsworth.gov.uk