Info provided is not very clear. Not sure whether your spouse-to-be still owns the existing flat.
I will assume the following:
1) Current flat own by your spouse-to-be already settled.
2) Your spouse-to-be owns the HDB flat at the moment.
Your choice is as follow:
1) buy an HDB flat with his name as an occupier, but he has to dispose of the current flat owned.
2) Stay in the current flat own by him and listed as an essential occupier.
3) Provided your spouse-to-be flat already met MOP, you may buy a private property under your name only. He can retain his flat under such a situation.
Hope the above answer to your main concerns, but if there are more query, please feel free to contact me at 90110636, or email: email@example.com if more information is needed.
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