Loft Conversions
Many modern and Victorian houses in London have large amounts of unused space under the roof. Simply adding a floor by boarding-out, altering the roof structure slightly and adding storage space in the eaves with a Velux window or two may be all you need.
The other option is a full-scale re-fit with the creation of bedrooms and ensuite bathrooms bringing with that complete plumbing and electrical solutions. Planning permission may not be needed whereas building regulations are always adhered to.