How do I include a legal clause on split utilities?

[Casity Kao]

We have a single family home with two levels and will rent out to two tenants. Gas electricity and water are all one meter. Utilities will be under our name and we will bill tenants. Lower level is 950sqft and upper level is 575sqft. The split will be 60% for lower and 40% for upper. How can we legally include a clause into the rental contract to bill both tenants? Any recommendations for legal terminology we should be using is greatly appreciated! 

[Patrick Liska]

Casity,

Both @Account Closed are right on this one. you can have it in writing if both agree to the split based on square footage of the rentals and it could also be trouble down the line with each new tenant. I would see what it would cost to install separate meters and go from there, do the split if it costs to much. But here is my question, you say it's a single family home, are you sure that zoning allows for two family home there, i would check that first. just because it can be, does not mean you have the legal right to do it.