Floor plan design for a duplex, small plot, Bauhaus style

A few thoughts on this:

[*]It is unclear whether access to the financial resources exists or if the financial resources are available as equity capital. That is already a difference.
[*]Even if it seems unimaginable to many homeowners - why push to your limit when building your house if you don’t have to and can achieve the desired living situation differently?
[*]Available liquid funds may not make you happy, but they definitely open up a larger world of possibilities – and that is not only in consumption but also for matters of the heart.

My thoughts on this
- If funds are available, then another investment.
- If funds are not available, why take the risk? Then look now for a neighbor willing to take over their half.
- Building a single-family house instead of a semi-detached house does not mean that the entire sum has to be sunk into the single-family house. It allows more freedom in design and at least some garden/air.
- What does this have to do with consumption?

I see no advantages, only a risk that entirely lies with the builder.
Will the desired purchase price be achieved?
How long will it take until the sale?
Which neighbor am I getting? Here it is so tight that from the distance and privacy perspective, it tends more toward EWL.
Restrictions in the planning.

If a plan is already being made for an as yet unknown buyer or tenant,

... I don’t have to sing my eternal litany here "A semi-detached house has TWO halves" ;-) I like the concept of mirror-image identical parts, but I would allow the first occupants of the second half (whether buyers or tenants) to redesign the non-load-bearing walls.

The air space in the entrance area across 3 floors is really cool.

But it is also a climate and acoustic fireplace with effects that are not only desirable.

I can understand your impression and at the same time see no disadvantage in it.

Oh no, I don’t see any disadvantage in it either. However, in my mind’s eye I get poppers (Kim Wilde and David Bowie) in one half of the house and teds (Tracey Ullman and Götz Alsmann) in the other, all styled with the Stustustudioline from L’Oreal, and the gentlemen with leather ties. In the carport stands a Ford Scorpio :)

Then now find a neighbor who takes over his half.

Why look for a neighbor and waste time when you are already able to act?

- Building a single-family house instead of a semi-detached house does not mean the entire amount has to be sunk into the single-family house. It allows more freedom in design and at least some garden/air.

Why, if this design freedom is not needed?

- What does all this have to do with consumption?

It was just a hint that with money you can also move things that are not just about yourself. That can be very satisfying.

I see no advantages, only a risk that lies entirely with the builder.
Will the desired purchase price be achieved?
How long will the sale take?
Which neighbor am I getting? Here it is so tight that in terms of distance and privacy it tends more towards a row house.
Restriction in planning.

Clearly, selling the constructed half of the house is a potential risk in terms of price and time—and at the same time also an opportunity. One can deal well with risks; avoiding them is just one of several possible strategies.
You have good options with the neighbor. Whoever you don’t like can offer whatever they want, they won't get the half of the house. It’s quite simple. The potential influence is greater than usual.
The need for "privacy" varies. If a shared wall doesn’t bother you, it’s not a disadvantage either.
There is no restriction in planning if you like the left half exactly.

Just because I built a house on three plots and thus probably went the way you prefer doesn’t mean there are really good reasons for another approach. Different people have different preferences.

There is a Ford Scorpio in the carport :)

It is so ugly that it’s cool again. Politically correct and completely converted to electric regardless of any economic sense...

I see no advantages, only a risk that lies entirely with the builder. Is the desired purchase price achieved? How long does it take until the sale?

You apparently start from what I consider a logical error, selling only after completion.

Restriction in planning.

I see the identical parts concept with only minor deviations as applicable differently than a 50:50 split - but one could also build fraternal twins, like see

You are apparently assuming the logical error in my opinion of selling only after completion.

No, I consider it an entry opportunity when prioritizing construction start. A buyer can also be found in parallel. But it does not have to be the primary goal.

The neighbor can look as appropriate as possible, but the habit of loudly celebrating every car race with buddies or the tendency to be a hoarder cannot be seen.

A wall where windows can be installed, fewer parking spaces that can be arranged differently, something in the building area that can shift the building may have advantages.

So for me, what advantages does a builder have who acts as a BU for the second half?