I have never heard of any specificationsfor calling a room an office or den but you are right about calling it a bedroom. You can actually call it a bedroom but should specify it is non-conforming. One other thought comes to mind when you say the garage was added later. Is it sheetrocked between house and addition? If not it is not a proper fire wall and will need to be finished off for safety. As long as it is mud and taped it should be fine as I have never had an appraisal call for it to be completely finished but that isn't to say it couldn't happen. I am not a certified inspector nor appraiser and this advice is from past experience. I hope this helps and if you have any other questions please feel free to call me at 231-0520.
Broker/Owner Real Estate Salutions of Great Falls