A new HDB rule to curb speculation in its commercial and industrial properties has been effective, said National Development Minister Khaw Boon Wan on Friday.

The average transfer fees for HDB commercial and industrial properties have fallen by about 33 per cent and 42 per cent respectively since the rule was introduced last October, he wrote.

Back then, the board had moved to ban new tenants from transferring their HDB rental premises to others for a cash premium, known as an assignment fee. Business owners "assign" their properties to others because they are either not doing well or to profit from a transfer.

While HDB has banned tenants who started renting HDB commercial or industrial properties after Oct 16 from transferring their properties, long-time tenants who qualify can do so until October 2016.

On Friday, Mr Khaw said that these tenants, who must have rented from the HDB for at least 15 years as of Oct 16 in 2013, will have the grace period extended to 2019.

This would benefit older HDB business property tenants, mostly retirees or soon-to-be-retired who have served their community for a long time.

About 7,100 long-time tenants, or 40 per cent of HDB commercial and industrial tenants, will benefit. Among them, 77 per cent are aged 55 and above, Mr Khaw said.

"I hope the extended grace period would better help them make the necessary business adjustments to dovetail with their retirement plans," he said. "This concession is a good way to recognise their many years of serving the local community," he added.