A growing number of Singaporeans are working abroad and simply consider their flats as investments.

They comprise the rising number of households that are sub-letting their flats. The figure climbed 21 percent during the previous fiscal year.

Between April 2008 and March 2009, the Housing and Development Board (HDB) approved 22,754 applications or 4,019 units more than the accepted applications last year.

According to property analysts, in spite of the present economic condition, such a trend is not necessarily because flat-owners are having difficulties in adhering to their mortgage repayments.

"The downturn happened in the middle of HDB's financial year, toward the end of last year. And while some households may have been affected, it's not enough to say they contributed to the increase – it's a variety of factors," said Nicholas Mak, a real estate lecturer at Ngee Ann Polytechnic.

One factor is that there is a growing number of Singaporeans heading overseas to work. Liz Choo, a property agent, told MediaCorp that over the last six years, there has been an increase of more than 20 percent in the number of flat-owners who approach her to sub-let their flats.

She added that most of them are heading to Vietnam, China, the United Kingdom and Australia.

Rents for flats with three bedrooms range between $1,500 and $1,800, while executive maisonettes, five-rooms and four-rooms can rent for $1,800 to $2,500 per month, depending on the location. According to property agents, a flat with five bedrooms located in the central area can rent for around $3,000.

The first increase in the number of Singaporeans who sub-let their HDB flats was prompted by the relaxation of the public housing policy in March 2007.

HDB loosened up its policy to provide greater flexibility for flat owners to monetize their flats when they retire or encounter financial difficulty. It will also expand the rental market of HDB flats.