Plot for construction next to existing building - "Single-family house in Grandma's garden"

My wife and I are dealing with the desire to build a house and are currently discussing what options we have.
One suggestion that is on the table comes from my wife’s grandma: "build in my garden, there is space!".

Well then, keep discussing. I see everything except space left in the garden. What is the square behind grandma’s terrace that has no border in the cadastral extract?
I too would theoretically consider a building plot most likely including the footprint of the current garage. Is it a shrine for the last car of the late grandpa?
Don’t be put off by the confusion about setback adoptions from the eastern row houses, I see no need for those there.

"With family eat and laugh, but never do business": from "grandma" I first read that the owner of the property (on which a building plot could certainly also be leased, for further details on this issue see "Pianistenthread" ) will probably pass away during your financing period; and with child and son-in-law next door there is at least one party besides or before your wife in the line of inheritance. What stops grandma from encouraging your wife’s parents to issue a termination for own use and move next door (because I actually do not even see real "space" for a "retirement house" in her garden)?

I don't want to dampen this enthusiasm, but again...
ASK the building authority FIRST. If they do not see a densification at this location, all considerations are in vain.

Think about the basic division, the rough! location of the main house. Try to comply with the prescribed boundaries and submit a preliminary inquiry.

If the municipal council does not want a house there, all efforts are in vain.
Basically, municipalities are interested in filling gaps. But the plot must also legally allow it.

I don't want to dampen this euphoria, but again… Ask the building authority FIRST. If they don’t see a densification at this location, all considerations are in vain. Think about the basic layout, the rough! location of the main house. Try to adhere to the prescribed boundaries and submit a preliminary request. If the municipal council doesn’t want a house there, all efforts are pointless. Fundamentally, municipalities are interested in closing gaps. But the plot must also legally allow it.

If you perceive this euphoria from me: rest assured. A forward-looking clarification is exactly what I want and why I started here this morning. @all Many thanks for the many responses and the aspects brought in. For me, the assumption is confirmed that the proposal a) requires some clarification and b) – in case of a successful clarification – the garage is possibly very unfavorably positioned. It is now up to us to decide whether we want to open this can of worms (clarification with the municipality, etc.) or let "Grandma’s proposal" remain a well-meaning but not further pursued suggestion. Thank you.

First of all, everything legal should be clarified: what could be done, what is allowed, what is not allowed, what must be complied with.
And you should acquire some basic knowledge.

That is why I am here. If I should (still) not have asked my question, I apologize.

What else can be seen in the photo? That looks like another house in the north?

What is the square behind grandma’s terrace that has no frame in the land register excerpt?

That is a covered firewood storage for her tiled stove.

I see here neither a "densification" (in the sense of the will of a land-use planmaker) nor a "gap" ...

This is a covered firewood storage for their tile stove.

... and if this storage can go away, also no worryingly reduced or exploitable site occupancy index.

or "Grandma's suggestion" to let a well-meant but not further pursued proposal be.

That would be my "Plan A" here ;-)

If I shouldn't have asked my question (yet), I apologize.

You're welcome. I would be against requiring a "driver's license for questioners" (or the certificate of a submitted preliminary building inquiry) here :)

It is now up to us to decide whether we want to open this can of worms (clarification with the municipality, etc.)

This is not even a can of worms, because building authorities are there, for example, to examine the potential of plots. Don’t be afraid, just inquire informally. Since terraced houses and multi-family houses are located in the immediate vicinity, the prognosis looks good. And honestly: selling or replacing a double garage is the least of our problems.