The link below is part of the governments Planning Portal website that specifically relates to extending your house without the need for formal planning permission.
We find the ‘mini guides’ at the bottom of the page on the link the most useful and easiest to understand.
Luke Williamson Architect offers this type of advice during our initial consultation. If your scheme does not require a full planning approval (Full Plans Approval), we would always recommend (and are able to obtain on your behalf) obtaining a Lawful Development Certificate. This Certificate can then be held with your house records confirming that the work carried out without planning permission was lawfully permitted under Permitted Developmentrules.
http://www.planningportal.gov.uk/permission/