HDB flat owners are actually leasing the flats from the Housing Development Board of Singapore (HDB) for a period of 99 years. So you can have an alternative source of income if you sublet your HDB unit. The rental regulations determined by the government of Singapore for subletting an HDB unit is detailed and complex so you need to be sure to go through the following checklist:

There are different regulations determined by HDB for subletting the whole unit or the rooms partially. As for renting out the whole unit, you should seek approval from HDB before subletting your whole flat. But for renting out a room in the unit you don't have to seek HDB's approval, it's obligatory that you register your tenant's details with HDB within 7 days. Also, your flat needs to be a 3-room flat or bigger, but for 1 or 2 rooms flat subletting of rooms are not permitted.

Only Singaporean citizens are allowed to sublet their entire flat. You need to fulfil the minimum occupancy period, which is for 5 years, before subletting your HDB unit. Finally, you need to seek the HDB's approval before you lease out your whole flat by submitting an application at HDB portal.