There are multiple threads discussing this in depth linked to from the presence faq. See posts 2, 3, and 4, for example.
But the short answer is that this is easy with some routers, difficult with others, so it depends in part on the features of your WiFi router.
FAQ: The Many Ways of Detecting Presence
That said, however, in other threads you described your use case as wanting to distinguish between two different floors in the same building. Are you not still in WiFi range of your home when you go to your office and vice versa?
If you are still in WiFi range of your home router when you go to your office, you can’t use WiFi as a presence method since you will not be marked as “away” when you go from one to the other.
But it just depends on how far away your router can detect your phone.