Do you mean that the building is a literal item of power? Or a separate object itself?
If the bartender becomes stronger just by being inside it ('behind this bar, I am god!') then an item of power is appropriate. I'd probably allow a +4 or +5 as the one-time discount due to the size and inability to move it.
If the bar itself is an animate object (haunted, genius loci, genuinely alive) then I'd skip the item of power rules and create it as an NPC, in the same way a characters wife, kid or pet would be. Then for information you could treat it as a roll on contacts to see if the bar knows what you need.
Alternatively, you could treat the bar as part of your character, it could be your own little corner of the nevernever, almost entirely separated from the rest of the nevernever (just don't try to use the cubicle in the ladies loos that says 'out of order' on the door) and the entrance is a permanent gate. That way you could take the Demesne power and have all the rules for using it there. For information you could perhaps tie in divination rituals in some way, either take ritual yourself and use your demesne to create aspects to use in the divination (scrying pools, runes etc).