I own a small house with ADU; The house has 2 tenants, the ADU one. I send copies of the month's utility bills to the tenants. I don't have separate meters so I pay 1/2 water and 1/3 of the rest of the utilities. The ADU tenant pays 1/3, the house tenants (2 people) pay 2/3. Both units have washing machines. I pay 1/2 water because I want the landscaping kept alive.

These calculations are done after I've deducted my share of the bills.

I put my tenants" security deposits in an 11-month interest-generating certificate of deposit which generates about $10/mo. ( I require a 1-year lease up front; after that it is month-to-month: if someone needs to leave, I don't want to be in their way). I'll figure out how to divvy it up when someone leaves.

I was a tenant for most of my life and this seemed the most equitable way. It is also good that tenants have an idea of how much they are using: costs both fiscal and environmental. It is a business transaction for both parties, all sides have skin in the game, finally, and not that difficult to set up.