Comfort dog request

[Matt J.]

Hello BP, I have a SFR rental in the greater Tacoma Seattle area (not in Seattle proper though). The existing tenant has been in place for about 6 months. When I rented to them they asked about getting a dog and I made it clear I would not allow pets.

They just informed me that their son has been diagnosed with autism and the doctor gave them a letter/prescription advising that he have a "comfort dog". 

Question - Do I legally have to allow this pet? 

As a parent myself I certainly understand wanting to help your children but the 20 year landlord in me has me skeptical, and frankly I don't like pets in my properties.