Generally, a two storey extension can be built 3 metres out from the original rear wall of a detached house. The width is determined by the width of the original wall or walls from which the extension is built. There is a useful and quite detailed guide to permitted development (PD) extensions which you can find here: http://www.gov.uk/government/publications/permitted-development-rights-for-householders-technical-guidance

Given that interpretation of PD rules can be quite complex – there are areas of uncertainty and a large number of criteria and provisos – it is always advisable to seek a certificate of lawfulness from the council prior to embarking on a major PD project.

Mike Dade (Build It Expert)