I don't know if it is illegal in Iowa, but who cares. Tenants always seem to know that the landlord is doing something "illegal". Red Flag #1
You are not obligated to rent to dogs.
Although sometimes cute, dogs are not a protected class.
You can require a pet fee, a larger security deposit for accepting the dog or even a higher rent because you do not want pets. The tenant can then take it or leave it.
