Apartment conversion

Financially that makes no sense, and in other ways too. If an average 2 bed apartment is worth 400k, a 3 bed duplex of the same standard might be 500k. I dont see you getting permission to drill a hole in the floor for a stairs anyway. The floor is part of the structure.

It is possible to join or split apartments that are next door, but then you have to deal with kitchens and bathrooms etc. And again, a 4 bed will never be worth the 800k you put in.

Easier to move out.

You apartment conversion is way off. Two 2-beds would be far bigger than a 3 bed. likely two 1-beds would become a 3 bed. Which is much closer is value.

I’m not sure why you think they wouldn’t be able to open the floor. They happens all the time in commercial projects. Pretty simple.

As it happens, recent finished a project to combine two apartments into one. Although not duplex’s, was two sub penthouses sure by side.

I didn’t say it would be worth as much. I said your area was off. It’s pretty straightforward .

Two 2 Beds will be much bigger than a 3 Bed. I mean, on bedroom count alone there’s 4 bedrooms, and that’s before the extra living and kitchen area is repurposed.

An Im not sure why you’re comparing it to a standard duplex. A standard 3bed is ~100sqm. Two 2beds will be ~160sqm. Being significantly larger and gavin 4-5 bedrooms will be reflected in its value.

IMO a more realistic conversion is 2x50sqm 1 beds into a 100sqm 3 bed. Perhaps not an increase is value, but much much closer - the construction cost is the loss too

If you only consider at the money. Your comparison was still wrong because you based it on 2 bed to 3 bed costs. If you correct that it changes everything above the scale of cost.

Why build a 4/5 duplex, buy a house for the same or less.

OP didn't mention size. Combining 2 x 2beds came from you. I've pointed out how huge that is and not typical 3 bed sized.
2 x1 beds makes more sense, as I've same a few times.

The layout will be awkward too, with windows and balconys in the wrong place.

Why do you think that? Most rooms would not be changing and to balcony and windows. And is not hard to solve for new spaces.

More space or bedrooms adds value but it diminishes. A two bed property could be 100 more than a 1 bed. A 5 bed will not be 100k more than a 4 bed.

Property value scales with floor area not "bedrooms". The jump from 2 to 3 or 3 to 4 is often bigger than 1-2 - as they will include more than just a single extra bedroom (additional bathrooms, storage, etc).

I doubt there are many 5 beds on the market tbh - Large penthouses tend to utilise the space for other purposes.
But regardless a 150sqm apartments are is worth much more than a typical 3 bed.

Two 1-beds to a 100sqm 3-bed probably still a loss, but a much smaller one.