You can't dynamically create tabs, unfortunately.
I would suggest that you look at using something like a two-portal setup: one portal listing the buildings, with an option to select a building to show the rooms in a second portal.
You'll need to add a new table occurrence "rooms__selectedBuilding" , related to "company 2" on a new global field where you'll store the building_PKid when a building is selected in the first portal.
The second portal will of course be based on "rooms__selectedBuilding".
You might also look at using a button bar and slide panel to create your "tab panel", with calculated names and hide whens, plus some arrows for when you don't have enough buttons in your button bar.
If this sounds like it'd work for you, take a look at the example file I posted on this thread: Re: Assign tab names from several records in a table