Loft conversion (roof extension)
A loft conversion for your house is considered to be permitted development, not requiring an application for planning permission, provided certain limits and conditions are met.
Important note: The permitted development allowances described here apply to houses, not flats, maisonettes or other buildings. You should check with your Local Planning Authority whether permitted development rights apply – they may have been removed by what are known as Article 4 directions.
Other consents may be required if your house is listed or in a designated area.
Wales: This guidance relates to the planning regime for England. Policy in Wales may differ. Contact your local planning authority for further information.
Details and more info taken from: https://interactive.planningportal.co.uk/mini-guide/loft-conversion/0
YourBuildPlans supply a building plans service to Builders and Building Companies across the UK.