Construction: What’s involved?

Basements take many shapes and can be of any size. Basement construction varies from cellar extensions to the formation of lower ground floors to creation of below garden rooms and large multi-storey extensions both below existing buildings and to the full property footprint.

Despite this wide variation, basements are divided into two principal types:

Open-site basements

These are the most commonly constructed in domestic gardens, brownfield sites where the existing property has been demolished or greenfield sites. In these circumstances there is ordinarily no adjacent structures requiring underpinning.
Due to the absence of adjacent structures, open site basements can be the least technically demanding and least costly basements to construct, with some construction and waterproofing techniques being available to open sites that are not possible in other basement types.

Beneath or adjacent to existing buildings

These are the most complex and demanding of construction projects as they require the combination of geotechnical, structural and civil engineering expertise. It also needs Health and Safety management to ensure that any risks, as well as the stability of both the subject property and adjacent structures are manage. These basements are often referred to as ‘retro-fit’.
When working in existing buildings, identification of the appropriate sequence for safety and structural stability is of utmost importance. It is therefore critical that suitably experienced specialist constructors, with an understanding of the engineering principles involved are consulted and included at the earliest opportunity.

Basement Construction

Cellar extension

This is the least complex form of retro-fit construction, generally consisting of underpinning to the existing walls extending the foundations downwards and increasing the height of existing cellars.
A new floor slab is constructed and appropriate waterproofing measures adopted to protect the new basement from water ingress through both the existing and new structures.

Vault conversions

Another growing trend in London is to convert the unusable coal vaults at the front of the property into a usable space. The usual process would be to turn the two vaults into one
habitable space. Although the work can be technical this is one of the best ways of adding additional rooms and square footage to your property.

Excavated retaining wall underpinning

These are ordinarily constructed below mid-terrace buildings, and vary from single to multiple storey in depth. The basement is confined in width to that of the existing building, with the shape and size defined by underpinning to boundary walls.
The new structure usually consists of underpinning, a new basement floor slab and may, or may not, also include a new roof slab to the basement at existing ground or lower ground floor. Also structural steel work to augment existing floors and internal bearing or tension piles is needed.

Piled underpinning

These are constructed where additional external space offered by semi or fully detached buildings enable the basement to be extended beyond the footprint of the existing property. These basements can also vary form single to multi-storey in depth.
The basement can be constructed to any shape or size within the boundary of the property. A piled underpinning scheme is developed to support the existing property whilst the basement is constructed. The basement outside the direct footprint of the existing building can be constructed using a variety of techniques.

In some instances the above types of basement are combined.

In every basement construction careful consideration of the appropriate waterproofing measures is of the utmost importance. The ground conditions, basement type and construction techniques dictates the type of waterproofing needed.

Construction: What’s involved?