I saw one unit condo duplex, originally is a double height ceiling for the living hall. i think the previous owner has re- structure and change the original living area (upstair) into another bedroom and bathroom.
can i know this need architectural plan and MC approval ?

the current owner dont know as he bought it from sub-sale.

No need

Depends on the renovation. Hacking of walls and beams definitely need MC approval. Even install aircon need approval. Installation of simple partition, shower glass scree etc. no need.

So he built a whole mezzanine floor? This one I thought would need approval. It has to be done with compliant materials to support the weight of the structure and humans above.

depends on management, but in most newer condos any major / minor reno will need management approval

By right need architect plan + approval.

1) must ensure materials and structure are safe to bear the load of people walking on it.

2) must make sure the building can handle the weight of the added structure and materials.

This is why perhaps older high rise are a mine field. Lord knows what sort of shoddy renovation has gone on which may potentially impact the whole building. At least with newer developments there is some oversight and some certainty with modern day best practices.

You also doing another renovation???

If already there.... Should be already gotten approvals previously.... If not, who dare to Reno like that without approval... And it's condo not some landed....