As the trend for residential basement conversions continues, the most popular question we get asked at Ensoul is "How much does a basement conversion cost to build?"


How much does a basement conversion cost to build? This is one of the most popular questions we get asked as the trend for London basement construction continues. However, home owners are now looking far more closely at the economics. Due to the increase in SDLT, extending their existing home can be a much more attractive option than moving.

Basement extension uk popularity

Interest in basement extensions throughout the UK since 2004


However, when you talk to a basement contractor, the range of basement conversion cost will vary from <£2,000 per sqr M to >£3,500 per Sqr M. The reason for this 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?
  • Type of basement contractor: Large firms have additional overhead and quality management costs. There is also their brand/reputation and how much spare capacity they have to take into account.
  • 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 terminology

Before we start talking about basement costs, here are a few definitions to help you navigate your way through the acronyms:

  • Basement shell and core: This is the concrete box without any fit-out.
  • Professional services: This includes Architects, Interior Designers, Structural Engineers, Geotechnical Engineers, Party Wall Surveyors, etc.
  • BS8102:2009: The definitive standard to follow for making sure that your basement is watertight.
  • Light-well: The escape hatch from your basement at the front or rear. As well as letting more light in, it can create an extra space for possibly an urban garden.

Calculating basement conversion cost

In broad terms, 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

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

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

Costs are typically £2,700 to £3,500 +VAT per Sqr M. This generally comprises:

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

Basement fit-out

Costs are typically £1,200 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,
  • The extent of expertise required from various Professional Services firms.
  • The quality of fit-out you require.

We trust the above has given you sufficient information to work out the approximate scale of budget required. 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