Tenants Bringing Own Washer/Dryer, How Much Extra Should I Charge for Water?

[Thomas O'Donnell]

I have new tenants moving in to my duplex which I live in the other unit. The water is not separated and I have estimated water into the cost of rent. However they stated that they will be bringing a washer of their own which I am allowing, and I told them rent would increase if this was the case. How much should I increase the rent to account for the extra water that will be used with a washer? I'd love to hear some suggestions/advice on what I should do. And yes, definitely will be adding it into the lease that there will be an extra cost.