Basement construction and groundwater in London advice

Basement construction in London has been a common for the last 300 years. However, the recent escalation in land prices and conservation restrictions regarding works above ground, have increase the desire to install new larger basements.

Such large basements are becoming more common, especially around Hampstead, Notting Hill and Chelsea.

A central document for consideration in the design of basements is BS8102 – The Code of practice for protection of below ground structures against water from the ground

BS8102 covers the use of:

  • a) waterproofing barrier materials applied to the structure,
  • b) structurally integral watertight construction; and
  • c) drained cavity construction.

It also covers the evaluation of groundwater conditions, risk assessment and options for drainage outside the structure. It applies to structures which extend below ground level and those on sloping sites.

There are many specialist water proofing contractors/manufactures that provide a guarantee such systems internal to a new structure.

This article therefore focuses on consideration of groundwater flows, assessment and mitigation outside basements.

Who should I get advice from for a new basement construction?

It the earliest stages in planning such a proposal you will need to form a team of competent professionals. These would typically compose an Architect, Structural Engineer and Civil Engineer.

The Civil and Structural Engineer can be from separate companies but should work together to form a strategy and scope of work for geotechnical investigation. Such investigation can be considered as essential, as it is critical to understanding the risks and design implications. Such work should be completed and enable a strategy to be set out before the Architectural design is progressed too far, to avoid abortive work or delays.

How long would assistance take and how much would it cost?

Sometimes we assist from planning to completion of construction over a period of 2 years. Other clients require out expertise for planning or for detailed design across 6 months. Basements are expensive and getting the right advice and design is important. Some schemes we work on are lowering an existing basement for a few rooms, where as others have included space for 10 cars, a cinema, a snow room, a hair dressers and salon etc.

In very broad budget terms:

  • the minimum is likely to be £2,500,
  • planning and design stage £8,000 and
  • full civil engineering service £18,000

Why do I need a civil engineering consultant? I have a waterproofing company already lined up.

The two are completely different roles. A waterproofing company will have specialist experience of how to waterproof the basement and collection systems. There are however, wider issues to consider. Such as:

  • What type of water am I collecting? Surface / saturated groundwater / vadose ?
  • Where can it be discharged?
  • What consent do I need from the Sewerage Authority?
  • What is the water quality – is the proposal compliant with the European Groundwater Directive / Ground Water Act.
  • Does the basement obstruct the natural groundwater flows and is likely to cause flooding to the neighbours?
  • What about groundwater collection systems and French drains external to the building.

Basement construction and groundwater

Photo provided by Client DominVSGroup

What to look for in a civil engineering consultant?

The Civil Engineer should have particular experience of groundwater hydrology, ideally from both an academic and practical perspective. Can they give examples of other sites where they have designed groundwater mitigation works?

Many large basements cost millions of pounds. The cost of good professional advice is minimal. Such projects are bespoke and it is recommended to consider if the civil engineering consultant will provide the more focused service to your project that is required. A major project such as Crossrail is likely to require a major consultancy to advise on groundwater, where as a single house development may be better with a smaller consultancy, with Director level assistance.

I want professional advice on basement construction and groundwater – how do I move it forward?

Please include your full details and include all relevant background information, including plans, location, surveys, photos, planning conditions, or anything else relevant to the your proposals to