This is the most popular question we get asked at Ensoul as the trend for London basement construction continues to soar. The dilemma is that the range of costs quoted by different basement contractors will vary hugely which can be very confusing.
This can be due to the size and financial muscle of the building firm i.e. small vs large and well organised; what kind of insurances and warranties are offered; their availability and capacity – i.e. do they need to fill their book urgently to keep their teams busy or are they in high demand?
Calculating basement cost:
Essentially there are three buckets for calculating the cost of your basement project:
- Professional Services
Typically come in at 15%-20% of the total project cost and depends upon the extent of services engaged.
- 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/guarantees).
Typically 50%-60% of the total project cost.
This generally comprises the dig out and creation of your watertight concrete box: perimeter walls, superstructure steelwork and underpinning, concrete floor slab, roof construction, ground and foul water drainage and architectural glazing.
- Basement fit-out
Typically 25%-30% of the total project cost.
This is for labour and fixtures, fittings, equipment and finishes for all the internal elements of your new basement. Includes 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 e.g. timber floors can range between £20psm 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 or playrooms.
Basement extension cost drivers:
- The size and volume of your new basement is the biggest cost driver.
We calculate your basement construction (2) and fit out (3) costs by applying a calculation to the desired number of square metres you are looking for and the finished internal ceiling height. Floor space is governed by what accommodation you are looking to put down there and how generously you would like to size each area/room. We recommend a finished ceiling height of 2.7m but you must dig deeper than that to factor in insulation, services and what’s on top.
Note: When calculating basement extension costs, generally an under garden basement can cost slightly less than an under house basement.
- Depth. How many floors are you looking to put underground - one or more? Some boroughs only permit one storey underground; others are less restrictive.
- Type of soil/geological structure and water table height affect how your basement is constructed and the methodology followed (piled or shutter/under-pinned). This in turn affects the price of the basement shell and core construction (2).
- Site access for conveyor belts and removal of spoil.
- Location of drains and mains sewers.
- Selections for your fit out (3).
Your personal basement estimate
If you're considering extending your living space with a basement extension, please complete the form below and we will create a personal estimate for you.