Creating a Basement to Add Valuable Space

basement design clapham

Excavating under your house can add valuable space, and its an option more and more of our clients are considering when looking at adding to the space they need. At Granit we have been working on new basements in London, and further afield for 20+ years.

Finding the space

Many of our clients need more space, whether its extra living space, bedrooms, or more dedicated rooms for entertainment like a gym, or swimming pools, or simply to create storage for M and E plant areas, utility rooms, or wine storage. With land at a premium, and properties typically occupying much of their plot, a basement can be one solution to meet these needs.

Design Considerations

Sometimes there will be an existing cellar or lower ground floor that can be exploited, but often a new basement will create a whole new storey to be created, which by its nature will be primarily subterranean. There is often a sweet spot between the correct floor to ceiling height and a basement which has too little outlook, as its too deep. One key consideration is how to maximise natural light into the basement areas, as this will make areas more useable and add more value. In our Briarwood Road project, we achieved this with front and rear lightwells, and an internal double height space flooding a basement kitchen with light. The compromise was a loss of part of the ground floor living space, but the benefits far outweighed this.

We tend to place bedrooms, and living areas towards the lightwells, with storage, utility and cinema rooms placed more centrally on the plan, but it does depend on the property and the opportunity of the site. For some uses, natural light is not a requirement, eg. a gym or cinema room, and the interior design treatment is more about artificial lighting, use of materials and soft furnishings. This where our interiors team really add value to a project. See more here.

Architects South West London

We have designed and implemented a number of swimming pools in basements, like our completed project in Hove. Here the biggest challenge was coordinating the complex air handling plant, and intergrating it into the fabric of the building in a discrete way, so that the interior space was not compromised. We worked with a dedicated pool supplier Newman Aqua.

Added value. Is it worth the investment?

20 years ago, it was only prime areas of London that we worked on, but in the last 10 years or so, weve seen an explosion across the capital, and surrounding areas, fueled partly by a rise in property prices themselves.  At £3,500-6000+ VAT per sqm of added space, a basement is a serious investment. But if that property is now worth £7,500 to £10,000 + VAT per sqm for every extra sqm added, then the investment starts to stack up financially. If you factor in the cost of moving, stamp duty, and a limited housing suppply, then one can understand why so many clients want to consider it as a viable option for their home. With all our clients the return on investment is one part of the conversation, the other is the quality of property that we can create with a basement, and the value you put on a beautifully designed home in an area you want to live in for yourself and family.

Planning Considerations

Each council has a different approach to the process of validating and determining a basement application and its a constant moving picture, as new policies evolve. In most London boroughs we are now expected to provide construction management plans, which may include a preliminary structural design or method statement, as well as information on how many lorries will be expected and hours of operation. There may be a flood risk assessment, or soil testing required from a qualified professional. Usually the key planning issues of new basements relate to the location of new lightwells, or lower garden areas, particularly if they are to the front of the property. This can be more pertinent in conservation areas or if the property is listed, or considered a heritage asset. At our Astwood Mews Project in Kensington and Chelsea, we were not able to place new lightwells to the front of the house, so the light enters through an internal void, and walk on glazed floors.

Mews House Refurbishment

Construction and timelines

Building a basement under an existing property is a complex process, with complicated logistics in terms of propping the building and sometimes neighbouring buildings during the works, excavating soil, and sequencing the works in a fashion that is safe at all times. There can be an ingress of water from a high water table to contend with, like our project close to battersea Park, where continuous pumping was required througout the build. The process of excavation can be a mixture of mechanised work and hand digging, and then physically getting the soil out of the site and into a skip of grab lorry can be laborious. Whilst each project and situation is unique an experienced team of Architect, Engineer and Contractor working together can problem solve all of these challenges. Depending on the size and depth of the basement, a programme can be extended by 6-12 months by adding one to a build project.

Trusted Partners

Much of the sucess of new basement projects comes from working with trusted partners, consultants, professionals and suppliers who all have a proven track record delivering basements. We will always work with an experienced main contractor like Woodmans or Strongbase, but in some cases the contractor will also sub contract to an experienced basement specialist like The London Basement company. We only work with stuctural engineers who have experience of designing basements, but also a practical understanding of the sequencing of construction, so they can support the builder during the construction phase. Engaging a good party wall surveyor is also something we always reccomend, and we have key firms we work with to negotiate the party wall awards on behalf of our client.

Working with Granit

We’ve worked on almost every London borough over the years, and many of these projects have included new basements, as can be seen in this map of completed and live projects. If you are thinking of adding a new basement please contact us here: