A useful article here from homebuilding.co.uk.
Some key points:
1. Your neighbour always has a “Right to Light”, so make sure that you always consider your neighbour’s sources of light. Basically, don’t block anybody’s windows.
2. Don’t sacrifice space to make space! If you are planning an extension, don’t butcher one of your existing rooms to allow access to your new one. Make sure you make best use of the space
3. Converting your basement won’t chip away at your permitted development rights (limits the amount you can extend your house), so it won’t stop you adding an extension down the road (dependent on other factors). Building basements from new is expensive, but conversions can turn an undesirable space into a new bedroom, living room, utilities area or clean storage area.