An individual with a minimum of four properties (commercial, residential or mixed) can treat the rental income as business income.
I have not been able to verify this anywhere else. Can you please verify?
In the past, there was a concession by the IRB that if you had four residential properties that were let out, you could treat the rental as business income. However, that concession has been clearly discontinued for individuals with the issuance of Public Ruling 1 of 2004. In fact, the IRB has said that it was never meant to apply to individuals.
However, the group treatment (which is not the same as the business-income treatment) has been confirmed to apply to individuals in the said public ruling. The group treatment does not stipulate any minimum units of properties but the properties must fall into any of the three categories (residential, commercial or vacant land).