For all HDB BTO flats, the main applicant must be a Singapore Citizen.
If the co-owner is a Singapore Permanent Resident (PR), then they will be able to apply for BTO flats of all sizes under one of the schemes they qualify under – such as the Public Scheme, Fiance/Fiancee Scheme.
If the co-owner is not a Citizen nor a Singapore PR, then they can still apply for a 2-room BTO flat in a non-mature estate. However, the non-Citizen must be holding a valid Visit pass or Work pass at the time of application.
For more information about the other eligibility criteria (such as income ceiling, age and property ownership), you can refer to this article on DollarsAndSense.