If you were building a garage from scratch,

[WinkyWinkola]

you may as well put in a bedroom + shower in the roof?

If you can get planning permission and can afford it, why not. Could always be used as a kids den if not used as a bedroom.

Do you need to go back to planning if you have the garage plans approved without bedrooms? I mean the exterior wouldn't change.

I'm not sure about planning but you would require additional building regs.

You'd need deeper foundation and more services running across ie central heating.

And insulation.

Brilliant. Thank you.

I'm actually considering letting the rooms.

the cost will be a lot higher, of course, but if you can afford it, it would also be a useful service flat for your driver.