How to plan and design an extension

Forget moving – make the most of what you have by extending your house up, out or down...

Basement conversions

Basement conversions are the most expensive and specialised projects to undertake, but a good idea if you live in a very high-cost area. ‘Conversion companies will charge around £250-£300 per sq ft to create extra living space under your existing home, and a 20 x 20ft basement in a London terrace can cost anything from £100,000 to £120,000 to convert,’ says Michael O’Flynn. ‘However, it can provide a whole new floor, and internal living space in the most sought-after areas can cost more than £1,000 per sq ft, so the overall gain can be well worth the investment’


‘The majority of conversions we do are very much about leisure,’ says Maggie Smith of The London Basement Company. ‘We create big family rooms with built-in plasma screens and audio equipment. It’s important to remember to incorporate good storage, as well.’


  • Older period properties tend to be more suitable as they have sturdier foundations, and many already have cellars, meaning foundations are even deeper. More modern houses can sometimes be converted, but check with an underpinning specialist to assess feasibility and cost.
  • If your house is a terrace, you may need to underpin your neighbour’s property as well as your own, and will need building regulations approval to cover damp-proofing, ventilation and wiring. Planning permission isn’t usually necessary for cellar conversion, but is for a new basement.
  • If a house has a timber floor, work can begin from outside, meaning you can live there during work. If the floor is concrete, work must start by going straight through, so you’ll need to move out for a while. You can tank a basement, but if the water table is too high, the cost will be prohibitive.
  • How long a conversion takes depends on the project size, but an average job of 50-80 sq m takes around 20-24 weeks.

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