Why landlords ask
Landlords sometimes ask for a SIN to run a credit check, but every major credit bureau can perform a credit check with name, address, and date of birth alone. The SIN is not required.
What to say
The OPC's position is that a SIN is not required for a tenancy application or credit check and any request for it must be optional.
Alternatives that landlords commonly accept are an employer reference letter, proof of income (paystubs, T4), and bank-verified rent payment history.
A landlord that insists on a SIN as a condition of renting can be the subject of a complaint to the OPC (or the provincial commissioner in BC, Alberta, and Quebec).
If you already gave your SIN
A written request that the landlord destroy it once a rental decision has been made is the usual step, and a complaint to the applicable privacy commissioner is available where there is a privacy concerns.