Basement Construction Costs in London

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

Are basements worth the investment? This article discusses basement costs in London.

As a headline summary, basement costs in London start at around £6,000 per square metre plus VAT. Basements require a significant investment in both time and cost, and they often cause significant disruption. However, the limitations on above-ground extensions in many conservation areas and sensitive locations in London mean that a basement might be the only viable option for gaining significant extra internal space.

Basement extensions tend to add a minimum of £8,000 to the value of a property per square metre. They may even add up to £20,000 per square metre in parts of west London. In purely financial terms, at least they can, therefore, be worth the investment.

Architecture for London is a RIBA Chartered Practice with a track record of successful basements in London. View our portfolio, or contact us on 020 3637 4236 to discuss your project.

Basement costs in London: living room interior design — A house in Canonbury, Islington

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 £10,000 per square metre plus VAT.

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.

Why do basement extensions cost more?

Above-ground extensions cost a minimum of £3,000 per square metre in London, whereas costs for building a basement start at £6,000 per square metre. Therefore, for the same area created, basement costs are double those of a rear extension. The high 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.

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

St Johns Wood Basement Construction costs — Propping before excavation, at our basement extension in St John’s Wood

Waterproofing – this may include using waterproof concrete or a waterproof membrane around the outside of the building.

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, 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, you must 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 costs in London: Georgian house basement bedroom — Our basement extension under a raised garden in Westminster

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 you may have to add mechanical ventilation.

Basement 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. Conversely, a basement gym would be a more cost-effective proposal, due to the simpler internal fit-out and finishes.

Islington house extension architect Georgian — A basement in Islington by Architecture for London, providing a new study.

Basement costs in London: consultants’ fees

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

Professional consultant fees – the total of these may be between 12-18% of the construction cost. These include architect fees, structural engineer fees and cost consultant fees.
A cost report, prepared by a cost consultant familiar with basement extension costs in London, will determine an accurate pre-construction budget for the works
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. The cost varies depending on the number of affected neighbours, budget for between £2,000 and £15,000.

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 — We received planning consent for a new build house with a basement, Muswell Hill

Pre-planning fees

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 £7,000+ 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.

Design reports

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 makeup on the site, which will help the structural engineer design the foundations and structure.

A pantry and wine cellar in the basement of a london house with oak shelving — A basement wine cellar in our Hampstead House project

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 effects of the proposal on neighbouring buildings. These might include overshadowing and internal daylight levels.
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.
Waterproofing design: this will depend on the location and the depth of the basement. The most common approach is a waterproof membrane to line the inside of the basement with pumps. Sometimes a combination of multiple methods is required.

Architect designed timber stair and kitchen in a london house with concrete worktops — A new timber and concrete stair to the lower ground floor in Brook Green

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 costs in London: construction reports

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 the greatest noise. It can suggest mitigating measures and recommendations to carry out some work during particular 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.
During construction, the approved inspector will visit the site and report on the works. They will comment on building regulation including thermal performance, fire and accessibility. In addition to the main staircase to the basement, you may be required to add a second means of escape, as an alternative way of exiting the building in the event of a fire.

Fee proposals for the above reports and surveys can be obtained from specialist consultants.

utility room in a shaker traditional style with joinery by plain english — A basement utility room with a skylight and a Plain English kitchen

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 in London (for a typical terrace house) start at a minimum of £6,000 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. These often contain car lifts, swimming pools, wine cellars, cinema rooms, steam rooms, saunas, light wells, roof lights and other expensive design features. A reasonable estimate across London might be £7,500 per square metre plus VAT and consultants’ fees.