I live in a 3 storey town house with an integral garage. I am considering a garage conversion to a sitting area. Can I change the existing fire doors on the ground floor to normal doors with glass windows to bring light into the ground floor. 

The change of use from a garage to a habitable room will mean compliance with building regulations, including delivery of a building notice to your council. Building regulations apply to ventilation, moisture proofing, insulation, fireproofing, escape routes, and structural soundness. As a result, almost any design decision must take them into account.

When dividing up the garage a new room is created. This room is subject to a set of building regulations that require an escape route and ventilation separate from the main room. Alterations such as an infill wall replacing the original garage door will also be subject to building regulations concerning the foundations. The building inspector will want to visually inspect windows, doors, fireproofing and foundations before he gives a certificate of completion.

Note existing fire doors would not be affected by the change - if all the doors were fire doors (rather than just the door between the garage & the habitable space) then this is not because of the garage - you could change them to fire doors with clear fire resisting glazed panels though.