Ideas for building a house on parental land

What are the dimensions of the house shown? Doesn’t look like a family could live in there :-(

Friends of mine have just realized it exactly as you describe it

They put up a house where the plot only had space for a garden shed???

I would first ask at the office

The aerial photo gives no reason to suspect there is potential for densification of the building area in the second row here. This rather looks like a development plan area built about forty years ago; these are not the typical candidates for that, nor do the neighbors have the lot depth for a second family house "in the garden."

The section of the street looks as if the building envelope is already well utilized and it also ends roughly in the middle there. In other words: building in the second row is not allowed for now.
So there seems to be a development plan.
The neighbor to the east at the corner also seems to have upgraded considerably. If he had been allowed, something would surely have spread on the property instead of adding two extensions/gables running across.
An extension seems more possible to me.
The development plan and the building authority will explain the details to you. I think a phone call will suffice.

I would first inquire at the building authority as well. Write them an email, attach the pictures you posted here. Give them the parcel number. I think they can quickly give you an answer as to whether it would even be approvable. If it then says that it is impossible, any further thought about it is just a waste. What are the dimensions of the house marked, and how much distance do you then have to the parents' house?

We built on the parents' plot of just over 700 sqm. However, we had the option to build at the end of the property and were able to use a walkway through a building easement agreement to access the driveway. We built a semi-detached house as a detached house. A distance of 3 meters had to be maintained. To the walkway, only 1.5 meters, since this was half taken into account in the distance area. On this side of the street, we are also the only house with rear development, not serving as an extension. Legally, it was possible. Would we do it again? Definitely not. Far too close to the parents/in-laws. To this day, there is still constant conflict because it still feels like the parents' property. Changes we make are difficult. Changes they make on our property are not discussed with us. I would like to have a fence, but it would lead to a rift. It simply does not feel like owning something of our own. This does not have to be the case for you, but it should be considered. It is also annoying that they are constantly standing on the terrace. No matter the time of day...

It’s also annoying that they are constantly standing on the terrace. No matter what time of day....

As is often the case, it can be both a curse and a blessing. I think it is very important to make clear and honest agreements beforehand and maybe even write them down on a piece of paper, even if that might seem awkward at first between each other. But....times change and people do too (oneself included). This should also include that everyone can do what they want in their own area, from the fence to generally being alone, strictly according to the motto: better to say the bad things right at the beginning. The problem with that is that you then also have to stick to things yourself that were always convenient before. Ultimately, it depends on the people involved whether that can work or not.