They might be bothered about the loss of off-street parking capacity.Planning departments I have found are not always quite so helpful but they are generally only concerned if you want to change the external look of the building. If you leave the garage door in place. and are just boarding inside then they shouldn't be too bothered for the use you are indicating.
The OP really really should not just assume that planning restrictions do not apply to this conversion - that could be an expensive mistake.