Aspergers, Cats and Accommodation

Helo all, I'm a 24 year old with Asperger's Syndrome and am currently in the process of moving into one of my local housing association's properties... One thing I made abundantly clear in my application was that I have a cat. Ive pretty much always had a cat in my life and consider them huge comforts and aides to me - the fact that there's a creature living in my house means I can't get too focussed on something as I have to feed it, clean it, pet it etc.

I've been told that I wouldn't be allowed to keep a pet in the flat I'm being given because it has a communal entrance (He's a housecat anyway, so he wouldn't be entering or leaving the flat, but that's irrelevant apparently) I was wondering if it would be possible to get some form of letter from the doctor saying that not having him would be detrimental to my emotional and mental wellbeing?