Fallout from inconsistent Pet Policy

[Jamil Barton]

I own a 3-unit property: a duplex and a carriage house. In the signed leases, I included a pet policy for the carriage house and the upstairs unit in the duplex. The final unit to sign - the lower apartment in the duplex - had a no-pet policy, which the tenant signed. 

Since signing the leases, the downstairs tenant has expressed several times that she would like to get a dog, which I have denied per her lease. However, she has since learned that the lease in the upstairs unit is allowed to a pet, and has stated that this is unfair and likely violates Fair Housing protected classes. In my area, that includes the following:

• Race, Color, Religion, Sex, Familial Status, National Origin, Disability, Military Status, Sexual Orientation, Age, Creed, Marital Status, Gender Identity, Source of Income, Immigration and Citizenship Status

Being 19 y/o, the tenant is feels that this is unfair treatment based on age. I'm not worried about being charged with a violation, but I would like to manage things consistent with FH protections.

As such, I decided to modify things so that none of the tenants get a pet. However, yesterday, I learned that the upstairs tenant has already put a deposit down on a dog. Learning this, I was inclined to allow all tenants to have a dog, but this upset the tenants in the rear/carriage house, who are convinced (based on previous experience) that the lower tenants will not take responsibility for the pet, affecting quality of life on the property (dog poop in the shared yard, etc.).

So, I've recognized that by having inconsistent rules I've gotten myself into a situation where I can't make all three units happy. Either the upper tenant loses their dog, the lower unit tenant can't have a dog at all, or the rear/carriage tenants are affected by the pets on the property.

My questions are:

1) Can I stand by original lease arrangements, where the upper and rear tenants are allowed a pet, but the lower tenant cannot?

2) If the answer to 1 is no, I think I have to stick with a no-pet policy. In which case, how best to handle things with the tenant who has already paid a downpayment on a dog, and who signed her original lease giving her permission to have a dog?

Thank you!!! and Please help!