Basement Conversion Types

There are a number of different basement conversion types so you have the flexibility to build the basement that best fits your requirements. The names are quite self-explanatory, and the largest difference between the different types is the square footage that you can achieve.

Under House (Retrofit)

Most basement conversions are under house retrofits, which means that we excavate a basement under a house which doesn’t currently have one. These come in two make sizes:

Full Footprint

A full footprint conversion covers the entire area of the house and can increase the square footage of your home by around a third. We excavate to the existing walls of the house, but the basement work will extend beyond those to fit light wells.

Half Footprint

A half footprint conversion is half the size of your home’s area – it provides less square footage but is cheaper to build as there is less excavation work and fewer building materials/days are required to finish the project.

The size of your basement conversion depends largely on your requirements, budget, and preferences. Some rooms may not require a lot of space, while others have minimum requirements – especially if you would like to build a subterranean garage or swimming pool.



Sub-basement conversions are excavated under houses that already have a basement, but would like to expand further. This creates a multi-storey basement and offers yet more room for expansion. In some instances, it can be difficult or impossible to arrange a sub-basement conversion, depending on what is underneath your home. If you live close to transportation links or there are essential water pipes under you home, you may only be able to have a single-level basement conversion.

Under Garden

Under garden conversions create a true iceberg home by excavating well beyond the walls of the home to create a basement conversion that extends throughout the garden. This means that the basement of your house will be considerably larger than any of the other storeys. It also offers more opportunities to get natural light into the basement since roof lights can be fitted to get light into various parts of the room, rather than relying on a single lightwell at the front.

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Case Studies

Loft Conversions Add a Fifth to Property Values

Loft conversions are one of the best ways to make your home more spacious – an intelligently-designed conversion can add as much as 30% more living space to your home without extending the footprint of your house. As with all home improvements, conversions also increase the value and sale-ability of your home. A study by […]

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The client wanted to make extensive improvements to her home in Wandsworth, including a loft conversion, a kitchen extension, and alterations to her existing lounge and one of the existing bedrooms. All of the work took around 12 weeks and they were extremely happy with the results.  


Avian Construction Ltd

Peterden House
1A Leighton Rd
West Ealing
W13 9EL

020 3174 1241