Building Without Knowledge - Doomed to Fail?

Well, now regardless of whether this is a good idea. Is there a development plan? Floor area ratio/plot ratio? I don't think this planning will be approved. Talk to the building authority first before you start building castles in the air here. It is also quite possible that the house will have to be newly accessed from the street. That will cost a lot of money. How old and in what condition is the main house (the mother's)?

Well, setting aside whether that's a good idea. Is there a development plan? Floor area ratio/floor space index? I don't think this plan will be approved. Talk to the building authority first before you start planning castles in the air here. It's also quite conceivable that the house might need to be newly accessed from the street. That would cost a lot of money. How old is the main house (the mother's) and what condition is it in?

The house was built in 1995 and is in good condition. Going to the building authority would of course be the next or first step. There are a few houses here where a similar additional house was built, so I am somewhat hopeful that it won't fail just because of that. But as I said - unfortunately - I have no knowledge of the topic.

I also think that the buildability would first have to be clarified here. In the shown section, in my opinion, there is certainly no justification from the neighborhood for a "Hinterhausbebauung."

Well, now, regardless of whether this is a good idea.
Is there a development plan? Floor area ratio/floor space index?
I don't think this plan will be approved.
Talk to the building authority first before you start building castles in the air here.
It is also quite conceivable that the house needs to be newly accessed from the street.
That then costs a lot of money.
How old and in what condition is the main house (the mother's)?

the first sensible answer!

First of all, one should check the development plan with regard to floor area ratio and floor space index. Where would a subdivision make sense, where is it even feasible.
What does the rear property offer, if it can be built on at all, in terms of floor area ratio and floor space index in square meters?

Regarding the costs: there is a lot more to come. It definitely won't stay at 150,000, maybe a frugal 300,000, of which you currently only have 15,000 equity. Too little!

I am professionally tied to the area and cannot imagine moving away because of “love”. Even if I did, the house could certainly be rented out. The square meter prices here are around 10€/sqm. I actually saw this as a kind of solution for the worst case.

You can’t be serious?! 10€/SQM?????????? If that’s true, I’ll buy your village! Then you don’t have to worry about a plot of land that probably cannot be built on.

And about the love... I’d better say nothing about that

Furthermore, it wouldn’t be house to house, there would still be a garden left. Even the building area of 85sqm didn’t seem that small to me, I was thinking about 60sqm ground floor + 60sqm upper floor - doesn’t that fit? By the 85sqm I mean the “space for the house,” there is still green space around it. On the sides, if you maximize the 85sqm, there is >3m distance. The total undeveloped area is 450sqm.

That’s not how a building authority calculates.

That can't be serious?! 10€/QM?????????? If that's true, I'll buy your whole village!

He meant 10€/sqm rent for apartments etc.

I wouldn’t do it like that either. Think about later, if Mom’s house is supposed to be sold, how is the access going to be arranged then? There can be quite a bit of stress with easements if the respective owners don’t get along. Or a division of the property? What would that then look like? You also have to reckon with the fact that the current residents (or in the rear house, the planned future ones) are not going to live there forever. A house has a somewhat longer lifespan than the usual (remaining) living duration of the people involved. Parents-in-law have something similar, front house and rear house, with a shared courtyard. The whole thing divided according to [WEG]. They’ve fallen out with the resident in the back and now there’s constant stress – who parks where in the yard? Can he just put up scaffolding there? What, they want to put a dormer in the roof? The garden is shared use too, or at least it was. If there’s constant bickering, how do you put up with that? I would never ever ever want to buy something like that. I would probably first ask if the rear property can even be built on at all and then prefer a proper division. Then sell the rear one and use the money to buy a new property elsewhere. By the way, we live 50 km from the parents-in-law and that’s really the absolute minimum distance I find just acceptable…