Make 2 out of 3 plots

First, it must be clarified whether the plots are within the scope of a development plan. This plan could contain regulations that do not correspond to your plans.

Concrete example: the plan allows one residential unit per plot - if you then combine two plots, it still applies to the larger plot - even if two semi-detached houses would have been permitted on both together before, a two-family house is still not allowed on the combined plot.

Hello, I will try to answer all questions.

The plots can also be developed with semi-detached houses according to the [Bebauungsplan].

The width is about 17m. The municipality wanted to keep the plots small because most people prefer smaller plots. However, the plots are quite deep (about 30-35m). You have narrow, deep plots. This leads to a dense development towards the neighbor and deep gardens.

Hence the idea to turn 3 into two. And 1 residential unit per plot. So on 3 plots, you would have 2 residential units.

I hesitate because I don’t know if I can manage it.

Regards

The width is approximately 17m.

For one plot or for all three together? OK, from your other thread I conclude that it is about one plot. Here in the west, people have different ideas of small narrow deep plots than 17m*35m=595m², namely e.g. 5.5m*40m.

I'm hesitant because I don't know if I can manage it.

So if the development plan is not in the way (you can upload a section), the conditions according to #4 are met and the finances for all three plots are sufficient, there's not much to manage. If you are that uncertain, go to a publicly appointed surveyor nearby and get advice. He will also clarify feasibility with the responsible authorities and tell you the costs (my estimate about 3k).

...
According to the development plan, the plots can also be built with semi-detached houses.

The width is approximately 17m. Since the municipality wanted to keep the plots small because most people prefer smaller plots. However, the plots are quite deep (approx. 30-35m). ..,

Small is something else than over 500 sqm.
Could it be that you are misreading the development plan and that semi-detached housing is a MUST there? The structure of the measurements fits more with normal single-family house development.

Best regards in brief

I also see no problem now fitting a double garage on a 17m wide plot.

to fit a double garage on a 17m wide plot.

... or to place an 11 m wide house between two single boundary garages. Apart from the fact that I would like to question whether giving the two tin children the biggest rooms should be a priority in the choice of plot.