The government has recently confirmed changes to permitted development to allow larger single storey extensions. For a period of 3 years, between May 30, 2013 and May 30, 2016, householders are able to build larger single storey rear extensions under permitted development; 8m depth for detached properties and 6m for terraced/semi-detached. Permitted development only applies if the property isn’t listed or in a conservation area and there are various criteria. Guidance notes can be found here.
OPS have recently submitted an application for such a larger single storey extension … watch this space!