You can first talk to the tenant and let her know that you are allergic to cats. You can follow-up your conversation with a written letter (keep a copy for your records). If the tenant is unwilling to remove the cat after your informal discussion, you can issue a Form N7 as you live in the same house and provided that you have less than 3 residential units. If you have more than 3 residential units, you can serve an N5 notice and follow the process accordingly. The reason for the notice would be that the tenant has substantially interfered with the reasonable enjoyment of the residential complex by the landlord.