A garage conversion is an exempt development under Class 1 of Schedule 2 of the Planning and Development Regulations (2001) as amended. The floor area of a converted garage is not taken into account when calculating the total floor area of extensions to a dwelling.
If it requires Planning Permission, it will require a Commencement Notice without Accompanying Documentation.
If it’s a two-storey domestic garage, it requires a Fire Safety Certificate and, therefore, either a Commencement Notice with Additional Documentation, or a 7-Day Notice.
Note
If you are in any doubt, contact your local Building Control Authority.