I need to schedule a meeting with me, my boss, and a third person (at my boss's level). none of the conference rooms are available at the only time the meeting is good. Should I ask my boss before I schedule our meeting in his office?
To be clear, my boss asked me to make this meeting. so it isn't out of nowhere.
Should I ask my boss before I schedule our meeting in his office?
By all means do tell him.
It's his office, so its courteous and professional to ask beforehand if their office is available or if it's ok with your boss to use it. Besides, you will also make him aware of the current space limitations, something which he could help solve (perhaps he can get a special request).
Imagine if you come in some day to your office and find it's being used for a meeting... you would wonder why they didn't tell you in advance would you?
Never assume someone is okay with having a meeting scheduled in their office, even if they were the one that called for it.
There could be any number of reasons why this is inappropriate:
What do you do instead? Tell your boss what you told us.
"There are no other conference rooms available for this time, would it be alright if we had this in your office?"
If it works out, then that's fine - you've done your due diligence and you've respected your superior's space and time. If it doesn't work out - e.g. your boss doesn't want to use their space for the meeting - then the worst thing that could happen is that the meeting is rescheduled for a later time.
If you use Outlook/Exchange to setup meetings, I'd just setup the meeting in his office, make sure notifications are sent (including the location), then briefly tell him in a mail, via the meeting notification, a chat messenger or personally depending on how soon the meeting is and how reachable your boss, that you scheduled the meeting in his office for lack of other rooms. Ask if this is okay or whether you should reschedule to another time or whether boss wants to kick out someone from the official meeting rooms.
This assumes you know your boss's office and know that it is suitable to hold three people, your boss does use it occasionally to meet with people, and it has everything you need for a meeting, e.g. a whiteboard or whatever you might require. If you have doubts about that, ask first.
Basically it's about probability. The more likely you think the boss' office is an okay location for a meeting, the more you can first setup and ask questions later and the less sure you are, the more likely you should ask first before you do anything.
