Building at the in-laws' place

Hello everyone
After much back and forth, I am reaching out to you. We would like to build on the property of my parents-in-law. However, the building envelope is quite small and we were told that we need to submit a building inquiry (reference was made to Paragraph 34&35 of the Building Code)
Can you tell me why a large plot has such a very small building envelope and what the general chances are of being allowed to build outside of it?
Here is a photo of the development plan - the property is circled in green. It is not clear what kind of building area it is according to the development plan.

I assume it is WR 1, since the neighboring plot is this building area. - Excerpt from the development plan also attached.


In addition, I can view the cadastral data on Tim-online and the property looks quite different there, meaning the residential building area has become larger.


Here are the data from Tim-online


I would be interested to know how this works - is it possible that although the residential building area is so large (and the building envelope in the development plan - which apparently is very old - is so small) a building inquiry could be rejected?
Honestly, I don’t understand why there is a building envelope if the development is determined by the plot ratio and floor space index.

I would be very grateful for your knowledge. Because we would prefer not to sit down with a construction company, plan the house, and submit the building inquiry if it is hopeless anyway.

reference was made to Paragraph 34&35 of the Building Code

That's nonsense. The property is located within the scope of an enforceable development plan (ok, I haven't verified that yet), so §30 of the Building Code applies. Whoever does not want to adhere to the provisions must state reasons according to §31 of the Building Code.

Can you tell me why a large plot has a very small building window

Because the majority of the plot lies within protected landscape areas.

how the chances generally stand to be allowed to build outside of this?

That cannot be answered generally and is always dependent on the individual case. In your case, I see hardly any chances, as according to Google Maps there are still some building plots free in the area.

It is not clear which development area it is according to the development plan.

Yes, it is.

I assume that it is WR 1

That is correct. It applies to the area bounded by the Knödel line and differently colored usage areas.

In addition, I can view the cadastral data on Tim-online and the property looks completely different there, i.e. the residential building area has become larger.

The types of use in the cadastre have no planning law significance.

Could it be that although the residential building area is so large (and the building window in the development plan - which apparently is very old - so small) a building inquiry is rejected?

Yes.

Honestly, I don't understand why there is a building window if the development is defined by the floor area ratio and the plot ratio.

Because they are two different elements of urban planning with different objectives. On the one hand, the "extent of structural use" to control the settlement density, and on the other hand, the "buildable area" to control the location of the building structure.

How is the property supposed to be developed at all?

Apparently, it is accessed by a dead-end street - as far as Bielefeld even exists, of course

Dumpling line

Is that a technical term?

I had to research it too. But since there is even a Wikipedia article, I am now waiting for the first opportunity to casually mention it in conversation to enjoy the reactions.