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Can a tenant be charged a pet fee if they have a visitor who is accompanied by a service dog?

Generally, no, but there are a few exceptions. Most often this situation is covered under the federal Fair Housing Amendments Act (FHAA), assuming it's a regular type of landlord (a private business or person who owns at least four residential units, ie houses/apartments). Churches are exempt as are a few others.

Plain English version of the FHAA: http://www.hud.gov/offices/fheo/FHLaws/yourrights.cfm
(Scroll down to "Additional Protection if You Have a Disability," not quite half way down the page)

Legal version of the FHAA: http://www.justice.gov/crt/housing/title8.php

Short version of the relevant passage:

§ 3604. Discrimination in the sale or rental of housing and other prohibited practices.
[I]it shall be unlawful ... to discriminate in the sale or rental ... to any buyer or renter because of a handicap of ... any person associated with that buyer or renter.

In my own experience, typically all that is needed is a phone call to let the landlord know that a person with a disability will be visiting and will be accompanied by their service dog. This is NOT required, but is a simple courtesy that can save headaches on both sides. Landlords appreciate knowing in advance (when possible) that a service dog will be visiting because it prepares them to answer questions from other tenants along the lines of, "Why is it Susie Jones gets to have a dog and I'm not allowed?"

Here's a sample letter I made up for a tenant with a service animal requesting an exemption to a "no pets" rule: http://servicedogcentral.org/content/node/285 A tenant with a visitor with a service dog might be able to take some ideas from it and tweak it to work for their purposes if needed.