(das) Gebäude = Building (Please put the dots on the a, it makes a difference to meaning as well as pronunciation. If you can't find the ä on your keyboard, use "ae", that is the accepted alternative).
(der) Bau = Building works (I.e. an unfinished building that is still in the process of going up). Bau also means "warren", as in where rabbits, badgers, groundhogs etc. live.
First, Gebäude is a neuter word (das Gebäude), so in the nominative AND the accusative case it will be "ein Gebäude".
Second, even if Gebäude were masculine, it would still be 'ein' as the verb 'sein' (to be - sind, ist, seid, bin) equates two equal things, so the case stays nominative for both sides. Therefore Gebäude is in nominative case.
Hope this helps!
It can refer to both.
Ich gehe noch zur Schule "I still go to school" means that you're a schoolchild -- here Schule is a concept.
Die Schule ist abgebrannt "The school has burned down" refers to the building
And something like die Wiener Schule "the Viennese School" would refer to a kind of movement rather than a literal school.