It may be possible to convert your garage under permitted development by virtue of Class A of Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) England Order 2015 (as amended). 

Firstly, you should check if your permitted development rights are intact, and that there are no specific restrictive planning conditions relating to the garage use or removing your permitted development rights. 

A restrictive use planning condition can be placed upon planning approvals and can be found on the Decision Notice. It is advisable for you to check any planning permissions that have been granted for your property and obtain copies of the Decision Notice/s to ensure that your permitted development rights are intact and proposed use has not been restricted before you commence any works. 

It would then be advisable to check the householder technical guidance to see if you comply with the Permitted Development criteria – Class A will be applicable for attached garages, and Class E for detached garages. Please see the permitted development rights for householders technical guidance 

For formal confirmation you should submit a Certificate of Lawfulness Proposed Development.   

If you are intending to convert your garage for business use, in some cases it may not require planning permission. You can submit a Pre-application request to discuss your proposal further, alternatively you should apply for a Certificate of Lawful Use or Development (Proposed) to check whether permission is required. 

The application forms and guidance notes can be found via the following link Download a paper planning application form and guidance notes – Rother District Council  or you can submit your application via the Planning Portal at Planning Portal – apply for planning permission