Planning permission is not normally required. However, permission is required when you extend or alter the roof space and it exceeds specified limits and conditions.
A loft conversion is normally considered to be permitted development and does not require an application for planning but this is subject to some criteria. i.e a volume allowance of 40 cubic meters additional roof space for terraced houses or 50 cubic meters for detached etc.
The government planning portal website has loads of information regarding planning permission.
All the planning and building regulations process can be done by ourselves and is included in your quotation.
For the full list go to www.planninportal.gov.uk/permission