• Find a stationers or website selling Ordnance Survey maps at 1:1250 scale, they are copyrighted so need to be paid for. This will show you the boundaries of your plot, the outline of any buildings, orientation and local roads etc. 6 copies are usually provided to cover Applications etc.
• The Local Council may have a 'Plan Registry' archive, ie records of applications and drawings that have been lodged with them, you can often purchase a print-out. This may provide sufficiently accurate scaled drawings for sketch designs. Also ask your conveyancing solicitor/building society/estate agent whether they hold similar information - especially if there are doubts over identifying legal boundaries. And there's google earth...
• If you do not have accurate plans and sections then you need to carry out a 'measured survey'. An architect, surveyor or anyone can do it provided you are willing to rely on their results.... inaccuracies could cost you extra in builders' time once on site.
• Take the site info and relevant photos to the Local Authority Planning office with a preliminary sketch of your intentions and ask whether there are any particular restrictions they may apply. An architect can also advise and will know their local authority well.
• Many Councils have a design guidance booklet which will suggest your easiest route to a Planning Permission. But these are usually a bit behind the curve.
• A certain amount of extension is also allowed without Planning Permission as 'permitted development'; if you have a property which has not been altered since the 1940's then this may apply, the Planning department will advise. You can also find much good information on the Planning Portal.
• Make a note of any planning history, plus any past problem or refusals, this may indicate what you might expect to be refused or approved, and also highlight any inherent problems with the site.
• Make friends with your neighbours and be open about your intentions, preferably without scaring them.