Q. I share an office with a young lady who was recently hired. Often when I come into my office there is someone sitting at my desk. They get up as soon as I walk in, but should they be sitting at my desk when there is an empty chair near my desk.
A. You are quite polite to ask, but I suspect you know the answer to your question. No, no one should be sitting at your desk and getting your office mate to understand this should not be difficult, or cause friction. These kinds of issues are often where colleagues ruin relationships, or don't even get to the stage where they might become colleagues.
It sounds like you may have lost a private office, which is happening more as companies find real estate costs too significant to continue to provide private offices to as many staff as they once did. People may work in the office less, or fewer days and they are now relegated to cubicles or shared offices.
Your recently hired young colleague may not recognize or be sensitive to the change in your circumstances, and may not have ever had the pleasure of a private office. Perhaps her previous office mate and she had worked out an agreement for guests to use the most comfortable chair available.
What is clear is the two of you have not developed an arrangement that works for both of you, and it is possible to do that. You can start the conversation when the two of you are in the office alone. "Sharing an office is not something I have done before. I'm not sure if you have, but I wanted to talk through a few things to make sure we make it as enjoyable and productive as possible."
You might decide to start with how the office is arranged. Does it work for both of you? Is there a window which you share the benefits of, or is it yours, or hers? Recognizing the mutuality of the situation will make all of the negotiations work much more smoothly. If you are more senior, there should be some perks associated with that - first choice, etc.
You mentioned that there is an empty chair near your desk. Is there also a "guest chair" conveniently located near your office mates desk clearly positioned for a guest for her? If not, find one. You may both decide to go on an office scavenger hunt looking for improved artwork, matching or more comfortable chairs. Each of you should have your own guest chairs. If by some chance there is not enough room for two guest chairs, one should be positioned in a convenient location so that it can be brought close to either of your desks.
As you look for these additions to your shared office, you can say, "I'd like to keep guests away from my desk and out of my chair. The extra chairs should work, and if you need an extra, I'm happy to have you pull my chair away from my desk so my materials stay private".
Office real estate is very important to many people. Your colleague needs to recognize her impact on your space, and you will be doing the same. Companies can help by making sure the appropriate furniture is available for officemates.