Just had work on our new basement completed recently. I am an estate agent and had a mortgage surveyor value my house again. The new basement has added £500,000 to the property value. Not bad seeing we spent just over half that on getting it built. I've bought and sold a few properties over recent years and had 3 basements built before selling on. I find that loft conversions and extensions add roughly 50% on the money you spend on building them. If the loft cost £60k you'll get it back plus £30k on top in most areas in London. Basements add the most as they provide the most space and you can make them into a flat and rent it out. Homes with an already converted basement sell very quickly within London. The first conversion was a nightmare but the next 2 were better as I used
London Builders, most property developers in London use them, I think they work on individual homes also. They work from outside the house, digging away under the front window and accessing from there only. When the works done they make access from the house to the basement. They use a similar style for lofts too and go in from the roof till everything's done.