Lang and $ recommended for Pet adendums w/spec permission?

[Jon S.]

What kind of language and fees would you include in a lease regarding pets? I did not intend to allow pets. However, one applicant has a black lab, which I have also owned and had good luck with, and she seems like a very responsible pet owner and person. I am considering writing an addendum of special permission for her to have a pet. What might I include in that addendum and what fees should i charge for having a pet? The dog is an 18 mo old black lab, supposedly just past its puppy stage, but I know black labs are puppies for a few years. However, I also think they tend to be well behaved, obedient and non-aggressive dogs. Still...what risks am I taking? 

I have all tile floors, new baseboards and trim, new doors, new paint, new kitchen cabinets and new appliances. 

Thank  you