House construction on narrow land? 3-family house already exists

That certainly depends somewhat on the framework conditions.. the land belongs to you? You want to live (stay) there? Alternatives would be selling or just mortgaging and of course building elsewhere. There are already possibilities to build narrow houses. Dirk's suggestion is of course - if possible - the better idea.

What does one of your two neighbors say?

Is there perhaps the possibility to swap areas? Then two beautiful houses can be created.

Maybe the driveway is shared?

I also think Dirk's idea is great, but that will be difficult since I only purchased the 3-family house just 10 months ago. The people next door are long-established and older. They are not supposed to be very flexible. I will politely address the neighbors again, but I fear that we will not come to an agreement.

And yes, I want to live in the property myself. Originally, I wanted a bungalow, but two floors, where half of the upper floor would serve as a roof terrace, would also be conceivable. The house really has a beautiful view to the back (view downward with lots of greenery). Therefore, I would also want to build at the back of the plot so that actually none of the houses in front would be affected. So, do you think that a proper house cannot be built with only 4.5 m width and about 16-17 m length? Total living space up to 130 sqm + full basement and large roof terrace (40-50 sqm)

Of course you can manage something there.. you naturally have certain restrictions and have to make certain compromises. I would look for an independent architect. It is naturally more demanding than a plot measuring 25 m x 25 m.

You say that the plot gets wider towards the back - you could also build a wedge there.

No, it is wider towards the front, meaning at the entrance of the existing house. Towards the back, the 9.50 meters remain, but I have some ideas. It's a pity that the kitchen and living room couldn't be as wide as desired because of this. The bedrooms, however, would not be a problem. Do you have concrete ideas for these dimensions?

Something can definitely be made out of it, but 9.5 is really quite tight.

Assuming you don’t get either of the two neighbors on board regarding building encumbrances, you have to maintain at least 3m of living space on both sides.
So you’re left with 3.5m of width for the building.
Optimistically, with a wall thickness of 0.30m, you have 2.9m of raw building width left for the room. That’s really tight.
It should be clear that a large kitchen-living-dining area of 50m² or more is rather unlikely.

But I can imagine a house with clearly defined zones and generous glazing on the long sides.

A good architect can definitely make something great out of it.

I find it very exciting. But I would really talk to an architect. I don’t think you’ll get far under these conditions otherwise.