As the trend for London basement construction continues to soar, the most popular question we get asked at Ensoul is "How much does a basement cost?"

How much does a basement cost? This is one of the most popular questions we get asked at Ensoul as the trend for London basement construction continues to soar and the cost of moving house in the £1m+ bracket, primarily due to changes in SDLT, which has increased on average by >50%. Therefore, home owners are looking far more closely at the economics of improving their existing home, particularly creating additional space, rather than the upheaval and significant cost of moving home.

Basement extension uk popularity

Interest in basement extensions throughout the UK since 2004


The issue that anyone has when they go online or talk to a basement contractor is that the range of basement costs will vary from <£2,000 per sqr M to >£3,500 per Sqr M and the scope of what is included is vague at best.


To be fair, the reason for the wide cost range is due to several factors including:

Scope: Is this the cost for the “watertight shell and core” alone or does it include professional services and what about the fit-out costs?

Size of building contractor: this can influence the cost due to the additional overhead associated with larger firms (which may by the way result in a more professionally organised site), their brand value/reputation and how much spare capacity they have to take on your project

Insurances and guarantees: the kind of insurances and warranties offered can have a material impact on costs

Geology and access: without a site survey, it’s impossible to ascertain ground conditions, water tables, drains, access routes, etc. All these are unknown risks that are priced in to high level estimates.

Basement Cost Terminology

Before we start talking about basement costs, a few definitions may help you navigate your way through the acronyms that the industry has grown up with as follows:

  • Basement shell and core: This is the concrete box without any fit-out
  • Professional services: This includes Architects, Interior Designers, Structural Engineers, Geotechnical Engineers (these are the engineers that understand the composition of the structure under your house or garden)
  • BS8102:2009: The definitive standard for making sure that your basement is watertight
  • Light-well: The escape hatch from your basement at the front or rear as well as a way of letting more light in and creating an extra space for possibly an urban garden

Calculating basement costs:

In very broad terms, based on our regular conversations with basement contractors, fit-out contractors and professional services firms, the following are the typical range of costs for a completed basement project of circa 100 Sqr M:

Photo calculating basement costs

Calculating basement costs

1.Professional Services

These fees typically come in at £60,000 to £120,000 +VAT depending on the extent of services engaged and scale of project.

  • Architectural design and planning application
  • Structural design
  • Surveys such as geological, water table, drains and trees
  • Party wall surveyor
  • Interior design
  • Project Management

2.Basement ‘shell and core’ construction (including insurances and guarantees).

Costs are typically £2,700 to £3,700 +VAT per Sqr M. This generally comprises:

  • The dig out and creation of your watertight concrete box including perimeter walls, superstructure steelwork and underpinning, concrete floor slab, roof slab, ground and foul water drainage and architectural glazing

3.Basement fit-out

Costs are typically £1,500 to £4,000 +VAT per Sqr M

This covers labour and fixtures, fittings, equipment and finishes for all the internal elements of your new basement including: stud walls and room formation, flooring, electrics, lighting, AV, plumbing, tiling, sanitary ware, kitchens if applicable, wardrobes and built in storage, painting and decorating etc.

These costs are hugely variable according to the selections that you make; for example, timber floors can range between £50psm to £400psm. It is also driven by what you are going to use the space for. e.g. cinema rooms and swimming pools are far more expensive than bedrooms and playrooms.


In summary, the three biggest factors that dictate the cost of your new basement are the size/volume of your new space, extent of expertise required from various Professional Services firms and level of fit-out.

We trust the above has given you sufficient information to work out the approximate scale of budget required, and, if this is of interest, we would be delighted to meet you and talk through this in more detail.

You may also want to read our step by step guide to basement conversion Basement conversion step by step guide

Your personal basement cost estimate

If you’re considering extending your living space with a basement extension and would like a personal basement cost please complete the form below or contact Mike on tel: 020 3637 0700 or email