Is financing the construction of a house feasible?

If the plot allows it, then a semi-detached house

2x150sqm is just as expensive as 1x300sqm. You don't save on construction costs with a semi-detached house. It also has to be financed first. If I then rent out the second half for €1,500 but have a €2,000 installment, I definitely can't afford the semi-detached house if it's already tight with the single-family house.

Construction costs are only 30% higher than for a single-family house.

How exactly did you calculate that?

2x150sqm is just as expensive as 1x300sqm. You don’t save on construction costs with a semi-detached house. It also has to be financed first.
If I then rent out the second half for €1,500 but have a €2,000 installment, I definitely cannot afford the semi-detached house, especially if it is already tight with the single-family house.

How exactly did you calculate that?

Basically, you are right. I interpreted it a bit differently:
Single-family house with 150m² around €500,000
Semi-detached house with 2x150 m² around €850,000

The calculation is of course more complex. Possibly double KFW funding. Rental income. Consider tax/depreciation.

Basically, you are right. I interpreted it a bit differently: Single-family house with 150m² approx. €500,000 DPH with 2x150 m² approx. €850,000

Oh, I see. So land €150k House 1 €350k House 2 €350k

Oh, I see.
So land €150k
House 1 €350k
House 2 €350k

If you build both houses at the same time, you should be able to save something. Only 1 building plot, 1 excavator, 1 crane, 1 delivery of materials, 1 trip for craftsmen (for those who only come for a few hours), maybe only 1 heating system…

If you build both houses at the same time, you should be able to save something. Only 1 building plot, 1 excavator, 1 crane, 1 x material delivery, 1 x craftsmen trip (for those who are always only there for a few hours),

That is actually negligible. One excavator and one crane at the same time fits (then you save €1,000).

maybe only 1 heating system…

Theoretically yes. Practically not feasible if there are two houses (too complicated). And if you ever think about selling, it becomes a dealbreaker. The savings you actually notice is the land purchase cost. The only reason why a semi-detached house/duplex is cheaper (apart from the approx. €5,000 savings overall, which you don’t notice in the calculation of a three-quarter million).

And until such a second half of a house or [Einliegerwohnung] is rented out, you have twice the costs. I would definitely not want to plan or do that even remotely. [Einliegerwohnung] is still okay for close relatives (care of parents, etc.) if you can afford the vacancy. But even from that, I can only strongly advise against it.