Hi TooManyCooks, we have helped a number of residents in SW London with this type of project, there are a few things that would need to be considered if you want to increase the height such as drains surveys, water table level, construction method, cavity drain membrane system design, etc. If you'd like to discuss, please call me on 020 3637 0700 or email
mike@ensoul.co.uk
Thanks, Mike Lander, Director, Ensoul