Inheritance share, family property, expand living area

Thank you for your information. However, I was thinking of other, further information... we seem to have misunderstood each other a bit or I misunderstood your question :(

So if all outbuildings are demolished, then you have an additional 105m² garden/building area

What would be necessary to know here, for example, are the dimensions of the plot, what remains at the top and to the right on the plan. Whether there is even enough space for an adequate house if you want to explore options. Whether the floor area ratio has already been reached, etc.

That also spoke in favor of expanding the barn, then structurally nothing would change.

Not structurally, but the usage conditions. It makes a difference whether it is a shed or living quarters. A shed may be built within the boundary distance, living spaces need boundary distances to the neighbor. Personally, I cannot imagine that if a closed development to the left on the plan was already intended (semi-detached house), then another boundary development by a residential house would also be allowed at the top on the plan.

Reuse of the shed:

Probably not.

Here I would have expected an answer that is more conclusive for the responder here, for example whether the shed is made of wood and whether it can be reused. Or masonry... Such a 'probably not' stops the line of thought. In our case, a shed is usually made of wood and is not a direct annex with the same roof tiles as the main house. In your case, it is rather a permanent annex. Maybe I also overlooked something here or rather suggested...

We want to do the upper bathroom.

So far, our idea was to buy it completely

The stove is a MUST. If that absolutely is not reasonable, then of course that can be discussed. But it would be very important to us.

My father wants her to visit him and stay overnight there. We do not want to accommodate her with us. Additionally, the 8 sqm is also valuable storage space. That is definitely a MUST.

Apparently, it is already certain that you will buy and build new for dad.

In any case, the architect did not see that as a problem.

What exactly do you want to get at?

Honestly, I thought you were looking for options here and it was not already decided that one of several mentioned options is almost certain. I thought that you want to get the optimum and the best for you and your father from the plot and existing building, but you do not know how to proceed or what is possible.

I did not read that you already have an architect. But maybe I overlooked that, or you just haven’t mentioned it now. Edit: Or I simply forgot it over the days (on re-reading I just saw it on page 1).
Therefore, I do not want to deal with the plot either, since it is already in professional hands. I have somehow seen the situation differently.

I think the problem is too complex to resolve it like this.
I still can't imagine anything at all under this shed and extension.
It does say something about use as a study/hobby room regarding the extension.
The barn looks like the continued use could at least be considered, but only professionals on site can decide that.

I also find the issue with building on the boundary very questionable, as to what is possible there.
But the architect on site knows much better about that.

How often does the sister come and why can she not live with you?
The 10 sqm for the guest room could make a huge difference whether there is space for anything or not.

Overall, I still can't see the win-win situation.
In the end, you're getting a pretty old small house for 400k. Except for your dad, this bungalow is also somehow wasted money (at least in my eyes).
None of the children will want to live long in a 50 sqm bungalow and finding someone else will not be that easy either.

Wouldn't it be an option the other way around, if your dad or your sister would pay you your share and you would at least have some equity?

Just get a quote from a local construction company that also does renovations!
Also, I would have turnkey offers made for your desired bungalow from prefabricated house providers.
Add demolition and 40k incidental construction costs to that, then you'll know what this option would cost.
So far you get it for free.