Landlords have every right to ask for SI number. Tenants don't have to provide it since it's optional under federal law. However, landlords have to check out the tenant and if a tenant can't provide any appropriate ID for landlord's to do a credit and/or other checks, then the landlord has the right to reject the tenant. There are so many dead beat tenants out there today that landlords are tightening up their checks and rightly so. Many prospective tenants don't have a driver's license, or at least claim not to, or any other appropriate ID to do a proper check so we resort to SI, if they can't give it than no go. The landlord has every right to reject a tenant if he/she can't determine the credit worthiness of a prospective tenant. Try doing a credit check on someone with the last name Chan, Mohammed, Smith or Brown, Khan, Rossi, Cruz, etc... with no proper ID. I was burnt on this over 30 yrs ago and it won't happen again.