Westminster house extension designed by architects, view of basement courtyard light well

Are you considering building a basement in London but unsure if it’s worth the disruption and financial investment? In this article, we will provide you with a comprehensive analysis and breakdown of factors that affect basement costs in London.

The limitations on above-ground extensions in many conservation areas and sensitive locations in London mean that a basement might be the only realistic option for gaining significant extra internal space. Basement extensions are, however, complex and disruptive, both for homeowners and neighbours. They also require a significant investment.

As a headline summary, basement costs in London start at around £4,500 per square metre plus VAT. As basement extensions tend to add a minimum of £8,000 to the value of a property per square metre (and up to £20,000 in parts of west London), purely financial terms at least, they are often worth the investment.

Basement cost figures will of course vary depending on various factors, including the specification, the site and the location of neighbouring properties. For very high-specification projects on difficult sites, basement costs in London may reach as much as £7,000 per square metre plus VAT.

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

We always recommend appointing a cost consultant (quantity surveyor) at an early stage to calculate an estimated price for your basement proposal. This estimate will be based on the architect’s drawings. An early price estimate check will limit the risk of submitting planning consent for something that later turns out to be unaffordable.

Islington house extension architect Georgian

A basement in Islington, north London by Architecture for London, providing a new study and guest bedroom suite.

Why do basement extensions cost more?

Above-ground extensions cost a minimum of £2,200 per square metre in London, whereas costs for building a basement start at £4,500 per sqm. Therefore, for the same area created, basement costs are more than double those of a rear extension. 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.

The approach to waterproofing will depend on the location and the depth of the basement. Sometimes a combination of multiple methods is required.


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.

Basement extension costs in London

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 fees, 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.

Architect basement muswell hill

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.

Building a basement: design 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.

London Basement extension architect

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.

Basement 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.

new house architect basement

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 Architecture for London’s portfolio to see Architecture for London’s recent projects.

Basement extension FAQ

What is the minimum basement extension cost?

Basement extension costs for a terrace house in London typically start at a minimum of £4,000 per square metre plus VAT. Consultants and planning fees will be payable in addition to the build costs.

What is the average basement excavation cost in London?

This is difficult to usefully measure due to the large number of ‘special’ basements in high value areas of London containing car lifts, swimming pools and other expensive design features. A reasonable estimate across London might be £4,500 per square metre plus VAT and consultants fees.