Unless this home office is going to have a bathroom, kitchen and a tenant , I'd probably just go for it.
If in doubt call the council but only if you want to risk not liking the answer. Unless you're running a business or doing something illegal etc it's your prerogative as to what you do inside your garage. That's my personal opinion.
Thank you No business. Exactly, I should say it should be a study room for me, and for practice music as well. My garage wall is built with double bricks. I want to add walls inside, with windows, need a door,too. Then I can install AC, make it comfortable in summer. Is it legal? or need council approve that?
If adding windows and doors ie cavities if they're not in place already, then you may need a CDC or DA. Call the council.