If the main problem with the size of your office is that you are embarrassed to work in such a tiny area, you will end up being even more embarrassed by bringing this up with your manager. The discussion will inevitably lead to the point that you should be spending more time thinking about your work and less time thinking about your work environment.
The only reason to approach your manager regarding a matter of this size is if your office is actually interfering with your productivity. For example, if people who visit you can hardly sit down, if there is no room to spread out important papers, or if people are getting injured by trying to angle themselves in and out of your office, then there is a legitimate reason to discuss this issue with your manager.
If there is nothing that either can or should be done with your office, it will be particularly important to remember that the most memorable part of your office is you. If you are embarrassed by the size of your office, your embarrassment is probably being noticed by those who visit you. However, your expertise, competence, and geniality can more than compensate for an office that doubles as a phone booth.
After all, there have been numerous sizable accomplishments emanating from miniscule offices. And, conversely, there have been numerous miniscule accomplishments emanating from sizable offices.
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