Can pets work in a duplex?

[Eric Littlepage]

After years of having bad luck with property managers and tenants, I took over as landlord of my 2-unit duplex (one on each floor).  I fixed up the vacant second-floor unit and rented it out last year to a young, couple, who have turned out to be the best tenants I've had so far--rent is automatically paid the first of each month, the place is clean, they don't bug me about fixing this or that, and they even fix some things on their own.  Now it turns out they want a dog, but I've always thought that would be a bad business decision.  I don't have a pet, but after the last few years and finally turning things around I'm just being very cautious, and I see a lot of ways this could go wrong--dogs tearing up the place, hair everywhere, going to the bathroom on the floor, poop in the yard... Also this being a duplex I have to think about keeping both units happy, and what if there's barking, or if a dog bites the other tenant... But it also seems this will be a deal-breaker for them and I'm afraid of losing my best tenants when the lease is up at the end of October.  They also say my ad said pets are ok with deposit and fee--I certainly wouldn't have put that in but maybe a box was checked accidentally. I'm trying to be open minded and am wondering if there's a way this can work.  If anyone would be responsible pet owners I would think it would be this couple.  They've asked if they can pay an additional pet fee, or if there's something specific I would allow, like size or breed or if a cat would be better.  So they are pretty open.  Having cats as a kid though, we had an issue with one of them marking his territory.  The duplex is hardwood floors or tile everywhere.  Just wondering if there's a good way to ensure success with this.  Are there certain breeds that are pretty quiet?  Has anyone had good luck reconsidering their no-pet policy?  Is a duplex just a bad idea to allow pets in?  Thanks for everyone's ideas!