Perhaps a pet deposit, and no mention of dogs or cats being allowed, just that all pets must be approved by you. The RIGHT dog owner can have a happy dog of any size even in an apartment (okay, as long as it's not a young, keep-me-busy-or-I'll-eat-your-furniture breed like a Border Collie!). I would go on a case-by-case basement, and you'll probably end up with grateful tenants and happy pets!