Building a mega-basement to double home's size

4 May 2019

(Mark Sim Photography)

Liam Dower went subterranean with a basement excavation for a recent residential extension.

Imagine doubling the size of your home. That is what I am asked to do regularly when I undertake the creation of brand new basements underneath family homes. The brief in this case study was to double the property’s size, through the addition of a bright and spacious subterranean floor.

I frequently talk to clients who live in great locations; they don’t want to move but have a growing family and need more space. They have over time extended outwards and perhaps into the loft, so they realise the potential of excavating the ground beneath them. Hidden underneath this period property is 100 sq m of additional living space, with a ceiling height of 2.6m and floor-to-ceiling windows providing natural light.

“Hidden underneath this period property is 100 sq m of additional living space, with a ceiling height of 2.6m and floor-to-ceiling windows providing natural light.”

Liam Dower

Basement design and construction is a specialised area with a high level of temporary works and structural engineering input. The ability to sequence the temporary engineering is critical.

The basement was constructed in a hit and miss sequence through a strategic arrangement of underpinning to the existing foundations. The formation level of the basement was around 3.8m and the lateral pressures at this depth needed to be accommodated for in the staged excavation to maintain the integrity of the surrounding environment.

This was achieved through temporary works design consisting of horizontal props at various key locations, modified as the basement expanded.

There was a large degree of structural steel installed to support the upper floors and associated internal wall structure. On the basement extending out into the garden we were required to divert drainage and install a reinforced concrete deck to house the patio area, hiding the vast living space beneath.

In excavating outside the building footprint we had to consider the external environment by insulating and waterproofing. Following the formation of the shell, fit-out works commenced with the installation of mechanical ventilation and pumping of waste water to ensure the space remains dry.

The initial excavation commenced over a period of six months including sequential underpinning, leading to the formation of the basement concrete shell. This was followed by a specialist waterproofing installation using waterproofing products by Delta Membranes, of which we are an accredited installer.

Natural light streams down the stairs into an airy space beneath the terraced house (Mark Sim Photography)

The fit out was to a high specification, working with the client to reach its aspirations for the space: including a media system with surround sound, a family wet room, extra rooms including a playroom, bedroom, utility room and home office.

One of the challenges was to bring light into the basement. We replaced the ceiling in the original hallway with roof light windows. The light now streams in.

This space has given the family longevity in this property. It is flexible enough to adapt as they grow.

My degree in construction engineering management formed the basis for my approach but it was not without its challenges. Taking the project from initial concept, through planning and design, then excavation and finally construction, is testing. A good working relationship with the client is essential.

My membership of the CIOB gave the client peace of mind it had chosen a professional with chartered building standards, alongside my experience in this challenging field. Over time, St Albans Basement’s reputation has grown, and we now work mainly by referral, often with neighbouring houses.

St Albans Basement is a Chartered Building Company, owned by Liam Dower MCIOB

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