Basement extension costs in London 2023

Basement extension costs in London typically start at around £4,000 per square metre plus VAT. This figure will vary depending on various factors, including the site make-up, the relative location of neighbouring properties, and the proposed use of the building.

For very high specification projects on difficult sites, basement extension costs in London may reach as much as £6,000 per square metre plus VAT.

This article explains the factors that affect the costs of basement projects. It can be used as a resource to help you avoid extra costs, enabling your budget to go further.

Architecture for London has a track record of successful planning consents for basements throughout London. View our portfolio, or contact us on 020 3637 4236 to discuss your project.

 There are significant extras to account for when calculating basement extension costs in London. Given that above-ground extensions tend to cost a minimum of £2,000 per square metre in London, we can see that basement costs (from £4,000 per sqm) are double those of a rear extension, for example.

Basement extensions are complex construction projects that represent a significant investment for the client. We always recommend appointing a cost consultant at an early stage to calculate a budget price for your basement proposal. This budget price will be based on your architect’s drawings.

A basement in Hackey, east London. This previously damp and unusable space was converted and waterproofed to create new office space.

Why do basement extensions cost more?

The significant extra costs of basement extensions are due to the additional construction and specification requirements as follows:

Excavations – digging out the ground for the basement and foundations and disposing of the materials off-site.

Temporary works – propping the existing building and earth during the excavations to provide temporary support.

Water proofing – there are several possible approaches to this, including using waterproof concrete or a waterproof membrane around the outside of the building. The most common approach is a waterproof membrane to line the inside of the basement with pumps.

Structural work – basement walls must retain the load of the surrounding soil. You may need to underpin other areas in the existing building, to ensure it is structurally sound in the long term.

Plumbing – depending on the existing drainage and plumbing, your new plumbing for the basement is likely to be more complex than an above ground extension.

Lighting – because the basement is either partially or wholly underground, it is particularly important that you consider the lighting requirements of the spaces. If specialist light management solutions are desired (e.g. Lutron) then these can add a significant cost.

Light scoops and double height spaces can improve the ceiling height and levels of daylight in a basement. These also allow the space to connect better to the rest of the house.

External works – these can be a considerable cost, with lightwells, landscaping and steps up to garden level, rooflights etc.

Ventilation – ventilation is particularly important below ground level. You may not be able to rely entirely on natural ventilation, such as openable windows and roof lights and you may have to add mechanical ventilation.

Access – in addition to a new main staircase to the basement, you may be required to add a second stair, or means of escape, as an alternative way of exiting the building in the event of a fire.

Use – many uses of basement spaces are inherently costly. A swimming pool is a good example, where this use has significant extra cost in excavations, finishes, mechanical and electrical requirements etc. Cinema rooms are also a common requirement in a basement. These can attract higher than usual costs if complex AV solutions are required in the space.

Consultants’ costs for basement extensions

In addition to the cost of the basement building work, you are also likely to incur consultant fees for the following:

• Professional consultant fees (architect, structural engineer, cost consultant, the total of these may be between 12-18% of the construction cost)
• Building regulations application – from the local authority or an approved inspector. (This varies depending on project cost and complexity, circa 0.5% of the construction cost). More information can be found on The Planning Portal.
• Party wall agreements – an agreement required with adjoining neighbours, to be arranged by a party wall surveyor (varies depending on the number of affected neighbours, circa 0.5% of the construction cost)

The cost of submitting a planning application to the local authority for a basement is usually £206. This assumes that the project requires a householder application only.

Remember that you are also likely to be liable to pay VAT on all professional fees.

Consultants’ costs: pre planning

Due to their complexity, planning applications for basement extensions require a number of specialist consultant reports. Basement extension projects, therefore, require a larger investment from a client at an early, pre-planning, stage than other house extension projects. In addition to architect’s fees, clients with basement projects should be prepared to pay £6,000+ to various other consultants for pre-planning reports.

The specialist consultant reports required for planning typically cover two main areas: site issues and construction issues.

Site issues

• A design, access and heritage statement is required with most basement planning applications.
• A basement impact assessment will identify and propose solutions to mitigate the risks caused by the construction of a basement extension.
• A flood risk assessment will help determine whether the proposal is viable, and how any risks can be mitigated.
• A SUDS (sustainable drainage system) report will help manage the drainage of surface water on site.
• A geological survey will include borehole tests or full trial pits to determine the ground make-up on the site, which will help the structural engineer design the foundations and structure.
• Structural report with design and calculations.
• A tree survey will identify any trees on the site or nearby that may need to be considered when designing the foundations and structure.
• Historical surveys and archaeological surveys are particularly important when dealing with Listed buildings or working in Conservation Areas.
• Sun and daylight reports will identify the effect of the proposal on neighbouring buildings in terms of overshadowing and the daylight levels inside the proposals.
• Drainage and services surveys will identify the current services in the ground. They also allow for the design of the new proposals to integrate into the existing works.

Warm, natural materials can create a cosy and relaxing space in the basement. Lower levels of daylight can even help to give a sense of seclusion – perfect for a yoga room or wellness suite.

Construction issues

• A construction traffic management plan will often be a requirement of the planning permission. This plan details the approach to site management and deliveries.
• A noise / acoustic report will identify the activities that will make greater noise and ensure these are carried out during the permitted hours.
• A dust / air quality assessment report includes environmental factors that have to be considered during excavation works. This will identify the types of neighbours that are nearby and how the impact on air quality can be mitigated.
• A vibration report is particularly important for basement projects due to the nature of the excavations required. It will identify the risks involved and propose mitigation methods.

Fee proposals for the above reports and surveys can be obtained from consultants that we often work with.

Constructing a basement is more expensive than a typical above-ground extension. If, however, there are site restrictions or planning issues with above ground development, a basement extension can provide valuable extra living space without the cost and inconvenience of moving house.

Please visit our project portfolio to see Architecture for London’s recent projects.

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