consider the following sentences
last night, I had a meeting in my office
last night, I had a meeting at my office
At the office or in the office clearly says both of them are correct.
Do they mean the same thing?
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In a colloquial sense, you'd say 'in my office' to denote the specificity of the location of the meeting, whereas you'd say 'at my office' to generally denote the general sense of having a meeting at work.
However, this is a personal interpretation based on experience. It should not be taken dogmatically.
I would use both of them is interchangeable in this sentience, but
at probably more accurate.
To know why, here is some explanation:
atto talk about locations at companies, workplaces when we see them as a place of activity: How many people are working at Microsoft?
into talk about locations within a larger area compared with the object: I know my book is somewhere in this room. Can anyone see it?