How can kids or pets mess this up? 

Same goes for pet deposits or last month's rent if you collect that in advance.    

.; you get the picture.I later learned when they moved they went on to purchase their own home and danged if they didn't advertise for room renters (and pets were allowed). 

What he ended up doing after pondering on what to do is be more Pro-Active thinking on selecting who lives Upstairs and Downstairs..Typically families w Kids or pets he rented downstairs, young Professionals he rented Upstairs.

I should be able to collect a "Pet Fee" & pet rent since it was technically a pet for the first month.

I discovered the pet while doing repairs last Friday.

SCAMMER steals the photos, makes their own listing...typically about 1/2 the price of normal rent and with some key words that turn people on...like pets allowed, no pet deposit, maybe no deposit, just 1st month's rent....something like that to make it attractive. 

Your no pet policy will probably be violated.

Does it have pet (dog, cat) pee after you remove the flooring?

Think of all the things that could go wrong and see if the lease addresses them: unauthorized pets or tenants, early termination, security deposit, lease violations, late rent, eviction, lawn maintenance, parking, etc.5.