From the 1st October 2008 changes to permitted development relaxed the requirement for planning permission for home owners to make improvements whilst limiting the size of an extension so that the neighbours could be protected.
Subject to certain exceptions, under the old legislation a home owner could only get either a small loft conversion or a rear extension without requiring planning permission. Anyone who had previously extended their property required planning permission for any further alterations.
The new rules now allow both and will be based on measurements for what is permissible for loft conversions and other extensions separately.
Rear extensions will no longer be calculated by volume for the purpose of permitted development. Loft conversions continue to have an overall volume cap to control overbearing conversions on larger homes. Extensions to the front or principal elevation will still require planning permission.
In conservation areas most extensions and alterations still require permission unless they are single storey and at the rear. For listed buildings the requirement for planning permission is relaxed but listed building consent will still be required.
There are some issues of interpretation in relation to the order, for example the definition of principal elevation, for which further clarification is being sought.
Always speak the relevant planning department before commencing work.