Think about banks. Do they have treasure chests behind the desk, or do they have big, locked vaults? As for the outside, Its basically a big stone box. Add some variation to the architecture, maybe some arches, bricks. The interior is colorless, try adding wool as carpeting in some areas (dark greens show wealth). Look up pictures of banks and notice the details on the inside, as well as the outside, like variation in the ceiling height, pillars, etc. Maybe even visit your local bank and just. Look around (try to arouse as much suspicion as possible).

its not done yet. im gonna add more stuff so itll look better!