2 different boundaries to the outdoor area

Then submit a building preliminary inquiry, anyone can do that themselves and it would cost us somewhere between 20 and 30 euros.

Simply mark the house in the desired position on a site plan and submit it to the building authority using their form. This will give you a binding statement on whether building is allowed there at all, regardless of the shape. Site plans are available at the land registry office if none are available.
If the result is negative (which one might expect given the previous statement), then you still have the option of legal action referring to the others. But it takes forever and success is not guaranteed.

Hello,

Backland development is allowed or rather must be allowed. In the second row, there are at least 4-5 houses in direct neighborhood.

"You can believe in church" – in questions around the topic of land and house construction, this emotion is less appropriate!

There are countless ways in which a clerk at the building authority can explain to you why backland development was allowed there, but not yours. Therefore, you should not initially insist that you can actually build on your parents' property.

The building authority... yeah, the good building authority. What can I say, we wanted to inquire how exactly it works, but were immediately treated rudely and directly rejected. With the words and I quote: THAT IS NOT POSSIBLE!

Then maybe the clerk had a terribly bad day, you perhaps slept a little worse the night before... etc. Usually, there are office hours for building applicants; you should therefore make a new attempt. For this, take a site plan with you (maybe your parents still have one) or obtain a cadastral excerpt for a fee; it doesn’t cost the world, maybe around €15 at most. Also, take pictures of the property and surroundings; make a simple drawing to note where on the plot you want to position the planned house.

If the clerk does not take care of your matter, is rude again, ask to be put through to the next supervisor. Just remember: always proceed/address in the way you yourself would like to be treated. Normally, the people at the offices are all very friendly and eager to answer the questions posed to them ;)

What do you mean by newly surveyed? and what should I understand by a suitable building envelope?

In anticipation of the answer: if you get permission to build in the backland, "your" part of the plot must be "separated" from your parents' land. For this, a surveyor is needed who takes care of the subdivision/partition. By the way, you need a surveyor anyway because he has to create the preliminary site plan if you can actually build. This site plan will then be part of the building application.

One more word about the formal building pre-application (BVA) – it costs at least €50 and often tends to be higher! What my predecessor describes is an "informal" BVA; this usually costs rarely more than €15. The disadvantage is that with this type of BVA only a "moral" commitment is made; it is not binding and no legal remedies are possible against it.

With the formal BVA, on the other hand, you receive a legally binding decision against which you can also file an objection if necessary. Creating it is more complex, checking it as well; therefore, the costs are often higher. A little sweetener: in my opinion, with a later building application related to the BVA, you can claim ⅔ of the BVA costs.

Rhenish greetings

The direct objection is basically correct, that only applies to the formal one. I would have had to elaborate further on the path to a lawsuit...

But I was also referring to the informal request, since anyone can create that without much help. When submitting a formal building inquiry, all documents must always be attached in three copies and an exact building plan must always be created. That is already more difficult for a layperson without help, so an architect may need to be consulted.
All of this causes costs. Perhaps the lady from the building authority also knows something (despite unfriendliness) that you do not know. A rejection would also be justified in the informal procedure, and then you can see what to do next.

For the surveying, the construction expert has already explained what I would also explain. ;)

Whether any development is possible at all could be informed by the existing development plan. The building plots (areas permitted for development) are marked there.

If this is not the case, there is the possibility to apply for a development plan amendment with the municipality or the building authority. In this case, the applicant bears the costs.
For this, the plot is considered by an urban planner appointed by the municipality from the perspective of possible development.

This is the path we took. Our building plot was only minimal and mostly in the shade.
Duration: 1 year, costs: almost €5,000 excluding surveying. That came to nearly €2,000 more.

I would first, before the preliminary building request, just look into the development plan as says, for example, how the building envelope is positioned. Development plans are now saved online as PDFs on the municipal websites.

Hello Stefan,

Rear development can be possible, but it doesn't have to be. The most informative here are mainly the standard land value map, the cadastral map, and the development plan or the existing development around if no development plan exists. It is also not mandatory to parcel the property, but parceling has some advantages. Without an up-to-date map, the existing buildings on the current property, and the planned development, little can be said about this in general.

Best regards
Dirk Grafe