What is the highest level of upgrade for a building?

[Pokgai]

And should I try to max upgrade all buildings?

[Shinichi]

I think for now it's level 5 and it applies to all buildings except tavern and guild house I think

[bombcar]

Upgrade the buildings that are cheap to upgrade, and upgrade the buildings you only run buffed. Compare the cost of the upgrade to the items received.

[Farfallarie]

Don't upgrade nobles past level 3 until you run out of room on your island (its cheaper to buy more noble deeds from a gem seller) and upgrade all your storage houses to level 3 before upgrading your Mayor's house to level 4. You get a more storage capacity for your resources.