HUD Handbook 4350.3 § 3-29

Some applicants or residents may require the use of assistance animals as a
          reasonable accommodation for a disability. (See the glossary for a definition of
          assistance animals).

          A.        An owner may verify that the applicant or resident has a disability and that there
                    is a disability-related need for the requested accommodation, in this case the
                    assistance animal.

          B.        The owner may require the applicant or resident to provide documentation of the
                    disability and the need for the animal from an appropriate third party, such as a
                    medical provider, mental health provider, or other professional in a position to
                    provide this verification. For example, if a tenant or applicant seeks a reasonable
                    accommodation for an assistance animal that provides emotional support, that
                    individual may be required to provide documentation from a physician,
                    psychiatrist, social worker, or other mental health professional that the animal
                    provides support that alleviates one or more of the identified symptoms or effects
                    of an existing disability.

          C.        The owner must implement its policy related to inquiries consistently for all
                    applicants requesting permission to keep an assistance animal. However, a
                    tenant or applicant should not be required to provide documentation of the
                    disability or the disability-related need for the assistance animal if the disability is
                    or the need is readily apparent or already known to the provider. For example, a
                    blind tenant should not be required to provide documentation of his or her
                    disability and the need for a guide dog.