Basement Excavation

A basement is a portion of a building that lies either partially or entirely below ground level. According to Approved Document B of the UK building regulations, Fire Safety, Volume 1 Dwellinghouses, a ‘basement storey’ is defined as follows:

“A storey with a floor that, at some point, is more than 1,200 mm below the highest level of ground adjacent to the outside walls.”

Basements can be constructed using various materials, such as brick, concrete block, poured concrete, pre-cast concrete, or even treated wood.

Typically, the cost of constructing basements increases with depth, although in prime locations, the value of land may justify multi-story basements or underground parking garages. However, this trend, particularly evident in central London with the emergence of larger ‘iceberg basements,’ has sparked controversy due to the significant disruption and disturbance they may cause to neighbours.

Efforts to address concerns about basement construction disruption, including the trend for ‘iceberg’ basements, have led to proposed restrictions such as the Basement Excavation (Restriction of Permitted Development) Bill and the Planning (Subterranean Development) Bill.

For instance, in August 2016, the City of Westminster introduced a new code of construction practice, which included an average levy of £8,000 on basement construction and the establishment of a new ‘subterranean squad’ to mitigate the impact of basement construction activities.

The construction process for basements typically involves several stages, including excavation and various methods of construction. Depending on factors like property size, excavation extent, and complexity, basement construction for a house usually spans 12–20 weeks.

There are three primary types of basement construction:

1. Poured concrete: This involves pouring concrete into formwork, resulting in a robust structure resistant to water infiltration.
2. Block and masonry walls: economical and relatively quick to construct, but less suitable for soil conditions prone to swelling.
3. Precast panels: panels assembled on footings, providing cost-effectiveness for multiple basement constructions but requiring careful sealing to prevent moisture issues.

Waterproofing is crucial for basement construction, with options including tanking, structurally integral protection, and drained cavity systems outlined in BS 8102:2009. Proper insulation and ventilation are also essential to regulate temperature and prevent moisture-related issues like mould growth.

Building a basement poses several risks, including drainage difficulties, poor ground conditions, obstructions, boundary issues, and waterproofing failures. Adequate site information and soil testing before construction are crucial for identifying and addressing potential difficulties.