Purchase advice: Land division, right of way & further planning

Since the seller's side has planned this for a long time and consulted with the building authority both in 2016 and now, I assume that development from the west will not be possible.

The "semi-detached house development" he proposed would actually border directly on the existing house.

There is a lot amiss. Only assumed by you, the owner will not disparage his own idea. I do not see that a semi-detached house development is possible here. I see detached development. Also, the building envelope may not be sufficient or there is a lot of garden land to the south. The brook may be some sort of association ditch; my parents have one too. I don’t know if that is an advantage or disadvantage. The access via his property is somewhat poor. However, could development be done from the west? It does not always have to follow the driveway!

That will not be approvable.
As far as can be seen in the picture, there is no rear development at the neighbors either.
The development is pointless and expensive, you can certainly count on an extra 50k there.
Münsterland is not exactly a high-price area (except for Münster itself), so something else should be found.

Neighbors still have no rear development, three houses down a existing house was demolished a few years ago and replaced by a large multi-family house. According to his statement, a developer did that and negotiated with the city for a year to get approval.

Of course something can be found in Münsterland, but we are currently limited to a few desired cities. Presumably people will become more willing to compromise over time.

There is a lot wrong. Only assumed by you, the owner will not disparage his idea himself.
I do not see that a semi-detached house development is possible here. I see detached development. Also, the building envelope may not be sufficient or there is a lot of garden land to the south.
The stream might be some kind of association ditch, my parents have one too. I don’t know if that is an advantage or disadvantage.
The access via his property is a bit miserable. However, could development be done from the west? It does not always have to follow the driveway!

He obviously has the development as a "semi-detached house" in mind, since this would only extend the existing building envelope about 3-5 meters to the south. In 2016 he also discussed this with the building authority and had it "signed off" on a plan there – though this likely has no legal value. I cannot say whether development of the utilities from the west is possible. Apparently it is not the access road.

I think we will basically express interest to him and state as a prerequisite that buildability is clarified. If that is the case, then I can inquire about development based on that. It should be standard that before buying a property, subdivision including buildability is clarified by the seller, or am I wrong as a layman?

Best regards
Malte

It should be standard that before purchasing a plot of land, the division including buildability is clarified by the seller, or am I wrong as a layman?

I think you are actually wrong there. He is selling you a piece of land (ok, at least the division must be possible), what you do with it afterwards... that's your business. I assume the draft contract will not mention the word "building plot." But it is already crazy if a single-family house doesn’t even fit on an 800m² plot. In this respect, it is exactly right, buildability must definitely be clarified before you sign anything.

Hello everyone,

to conclude the topic here as well: We actually had an appointment to clarify the open questions, but it did not take place. The seller has backed out and now does not want to sell after all. He assumed he could sell without capital gains tax – however, his tax advisor corrected that misconception.

For us, definitely another experience in the long search for a home of our own.

Best regards
Malte