Both Michael and Satchers are right re planning matters.
Do you have a window in your side wall? If so has it been there for twenty years? if answer is yes, then you will have "rights of light" which isn't a planning matter, but it means you'll have a legal right for that window to receive light! If you haven't put a window in your side wall, you probably wish you had! (sorry..)
If they build next to your house, or indeed within 6m of your legal boundary, they will need a party wall or party structure award. You should look up your rights here; http://www.communities.gov.uk/publicatio.../partywall