Not sure what you mean by 10%. Permitted Development rights for rear extensions to a house are governed by distance/height not % volume. You are not allowed to exceed 50% of the house's original curtlige. If an existing extension extends more than 3m beyond the original rear wall of a detached house, then you cannot build a second storey under PD (see page 19 of this guide to PD rights)... http://www.planningportal.gov.uk/uploads/100806_PDforhouseholders_TechnicalGuidance.pdf I couldn't see an example for a semi or terraced house, which tends to suggest Full Planning Permission would be required in such cases, but that's something you can easily confirm with a phonecall/email to Southwark planners...best to get what they say confirmed in writing. If you went down the FPP route, you should first ascertain whether the foundations of your single storey extension are strong enough to support the additional load of a second storey. A structural engineer can do this for you. My guess is that they wouldn't be, and a lot of additional steelwork to carry the additional load would be required, which might seriously impact on your budget...