You don't need planning permission if you are carrying out the alterations within the existing structure. However, if for example you put in a bay window which protruded beyond the building line (the existing extent of built development) you may require planning permission if it is facing a road.
If your house is less than twenty years old there may also be a requirement for your property to have a minimum amount of off-road parking space. By turning garage space into living accommodation you are reducing the off-road parking area, although you could offset this by making sure there was adequate parking elsewhere within your residential curtilage (your property area including all garden land and drive etc.). A rule of thumb is two spaces for a three bed house and three spaces for a four bed house.
You will need Building Regulations consent. This is a separate council department from Planning. The Building Regs. Inspectors are concerned mainly that the alterations you make i.e. new window or door openings, are structurally sound and won't fall down. They're usually very helpful if you ring them. Most half decent builders know the score anyway about applying for Building Regs.