Which stage does HDB lock in your income for BTO application?

Based on personal experience.. they calculate the grants and HLE based on two different timings...

Grant - On the date of application.

HLE - It expires every 6 months. Hence, when leading up to your key collection, you have to possess a valid HLE.

If I am not wrong, it's the time when you need to collect the keys. The first HLE (when I applied for the fiancee/fiance scheme), it was just to allow us to book the place. The subsidies are always calculated at the point of taking your house.

Best though you can check with HDB and ask "I am afraid my loan cannot approve" and ask subtly on the grant part instead :P