If you intend to buy a new Executive Condo from the developer, you would need to sell off your HDB flat within 6 months of TOP of the EC. In other words, if the new EC TOP is expected to be Dec 2014, you must dispose of your flat by Jun 2015.
This restriction applies only to new EC, for instance, when you buy during an EC launch.
If you buy a resale EC from the open market (executive condos that are above 5 years), then you would not be required to sell your HDB, as the resale EC is considered ‘privatized’.
Tip: To know what are the ECs that are more than 5 years, simply use our website’s Quick Search function. Choose ‘Status’ and select ‘Over 5 years’, our system will return you the executive condos that can be bought from the resale market (without having to sell your HDB).