Since increasing building limit might have impact on graphic performance, and also for more immersions & efficiency, i suggest option for upgrading plausible non-house building with 2nd floor that won't take any building limit, the upgrades will be:
- Tavern 2nd Floor Upgrade: Residence for 4 People Family
- Merchant Stall 2nd Floor Upgrade: Another Stall
- Kitchen 2nd Floor Upgrade: Mini Shared Granary (100kg)
- Tier I Workshop/Smithy/Tailor 2nd Floor Upgrade: Mini Shared Storage (250kg)
- Tier III Workshop/Smithy/Tailor 2nd Floor Upgrade: Another Tier I of it's Kind
Make the upgrades only available in the later game when you change generation (child, grandchild, and so on) and make the stairs facing the entrance so it won't cause common bugs...
If you have suggestions or critics, feel free to provide...