Building space: Basement conversion vs loft extension

Feb 28, 2023 | Property News

In a city like London, space is at a premium. Most homeowners have little room to build garden extensions or outhouses, so those looking for more space are left with a choice between a basement conversion or a loft extension.

Traditionally, Londoners have chosen to build upwards and convert their attics into additional bedrooms when looking to find extra floorspace to their property. However, a recent study by academics at Newcastle University’s School of Architecture, Planning & Landscape found that basements have become almost as popular in recent years. Between 2008 and 2019, the researchers discovered that 7,328 basement construction projects were approved by local authorities, with an estimated total of 1.8 million cubic metres of earth excavated from under the capital for their construction.

For those looking to add extra living space, bedrooms, offices, kitchens, gyms, or even a swimming pool in 2023 – would it be better to build a loft extension or basement conversion?

Basement conversion


  • You can choose how much space you want to create – many people may simply want to convert their current cellar into an extra bedroom or home office, but it is possible to excavate down two or three stories and expand the basement under the garden to create large amounts of additional floorspace if desired.
  • Multiple uses – basements are quiet and well-insulated, and so can provide you with extra space for bedrooms, a home office, a cinema room, a home gym, a swimming pool, and more.
  • Separate access – your basement could have a separate front door, making it a perfect office for entertaining clients or a semi-separate living area for an elderly parent or other family members.
  • Quiet and well-insulated – surrounded by metres of earth, a well-insulated basement will hold its heat in the winter and stay cool in the summer and should remain a quiet zone away from the rest of the house.
  • Luxury – basements are a perfect way to add luxury to your home, from a heated swimming pool and spa, to a cinema room or wine cellar.
  • Cost of a cellar conversion – if you do not need to excavate, converting an existing cellar into extra living space or a home office is relatively straightforward and a cost-effective way to add additional floorspace to your property.


  • Cost – if you do not have a cellar already or want to add more space and require excavation, a basement extension can be expensive.
  • Time frame – large basement extensions can take several months to complete due to the size of the excavation.
  • Planning permission – you may need planning permission if you plan to excavate and build beyond your current cellar.

Loft extension


  • Natural light and great views – at the top of the house, loft conversions can offer spectacular views and feel spacious and airy with a lot of natural light.
  • Cost – loft extensions are a cost-effective way to add extra bedrooms or a home office to a property if the plans are relatively simple, as they can be added with little structural change to the building.


  • Limited by existing loft height – some lofts will not provide sufficient space for a person to stand up, which may make a loft conversion difficult without significant additional construction, which may be expensive and require planning permission.
  • Limited by existing loft space – loft extensions build on the existing property floorplan, so most loft extensions will only have space for one or two rooms.
  • Planning restrictions – there may be planning restrictions to the height you can build.

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See previous construction projects in BatterseaChelseaClaphamFulhamKensingtonPimlicoSouthwark & Wandsworth.