Building height of 8.5 m with basement and 2 full floors

exactly how I determine "A" and "B"

You do not have to determine "A" and "B". Either your property is located on planned street "A" or on planned street "B". Unlike in your case, in my example the building plot is adjacent to two streets, so that the development plan would have to clearly specify which street is to be used. Let's assume "B" here. Then the center point of the street-side boundary (blue) is determined, at this point the center of the street (red). The height of the magenta-colored point is then the reference height. While the center for parcel 174 can be determined exactly, this is no longer possible for parcel 751, hence my criticism regarding the definiteness of the eaves reference point.

Where can one find information on how the procedure would be here?

If I see it correctly, in your case the planning authority (city) is not the approval authority (district). As soon as you have found an architect, they should give their assessment and inquire at the district office how such stipulations are handled there. If the answers are unsatisfactory, pressure can be increased step by step. The question would be whether the district risks a lawsuit because of the city's carelessness.
Regarding time and money I am clueless. I once read in some court ruling that led to the invalidity of the development plan of a dispute value of €2500. I know of a case that was decided after two years. But whether that is representative ...?

that the basement will definitely be completely underground

If I have correctly identified your property, your garden is about 1.5 m below the reference point on the street, so I see no obstacles to letting the basement protrude toward the garden far enough to install normal windows or even an exit. The architect has to work this out due to full storey and eaves height.

While the midpoint for parcel 174 can be clearly determined, this is no longer possible for parcel 751, hence my criticism regarding the definiteness of the eaves reference point.

Thanks. Now I understand. Because of the curve, it really isn’t that easy with my property either. But with a rough (re-)estimation on my part, it shouldn’t vary that much. I think, as you already wrote below, we are about 1-1.5 m below the reference point at the middle of the property.

Regarding time and money, I’m at a loss. I once read something about a dispute value of €2500 in a ruling that led to the invalidity of the land-use plan. I know of one case that was decided after two years. But whether that is representative …?

I have also heard from other sources that procedures are more likely to be classified as multi-year. Unfortunately, I don’t have the time, the kid is supposed to start school in 2019. And ideally right there, where he will grow up (so no school change).

If I have identified your property correctly, your garden lies about 1.5 m below the reference point on the street, so I see no obstacles to have the basement protrude far enough towards the garden to install normal windows or even an exit. The architect has to figure that out because of the full storey and eaves height.

I have sent you the exact property details again by private message. I also think it is between 1 and 1.5 m. Then I was initially right with my interpretation and can plan the basement – at least on the downhill side – looking out of the ground (windows for office and guest room and utility room as well as storage space completely underground on the uphill side).

I have contacted architects. We are also still open to building with a prefab house provider. Currently, Weberhaus is far ahead. Do they discuss such details with you or can you not expect that from the planner of a prefab house provider (= seller)? It’s really important to us that everything is interpreted correctly. Not that it becomes too much phone calls for a prefab house provider and then they start planning sloppily.. :/

Do they also discuss such details with you, or can you not expect that from the planner of a prefabricated house provider (= seller)?

The seller only wants your signature. There are no problems with the building permit on their part. The planning service is usually carried out by a regional planning office that follows a standard program. Various scenarios are conceivable: (1) You buy your prefabricated house with two full floors, a basement, and a 28° gable roof. (2) The planner prepares the building documents according to §61 Thuringian Building Code because, like the elephant and me at first, he assumes the ground floor slab is to be accepted. (3) You submit the documents to the municipality, (4) they do not respond for a month, and (5) you can start. Or (4) they say an exemption is required, switch to §62, submit a negative statement to the district office, (6) which nevertheless grants approval due to the questionable designation, and (5). Or (6) which denies approval. Your justified objection (7) is remedied, you receive the approval, and (5). Or (7) is not remedied. (8) You revise your plans, receive the approval, and (5). Or (8) you sue... Or (1) YOU clarify everything with the involved authorities beforehand and buy a house that complies with their interpretation. Intensive preliminary clarification taking deviation possibilities into account is not to be expected from the prefabricated house planner.

We are also still open to building with a prefabricated house provider. Currently leading is Weberhaus. Do they discuss such details with you, or can you not expect that from the planner of a prefabricated house provider (= seller)?

These are two questions: b) no, the seller does not plan anything; a) the timber framers do not build into the ground, you will probably have to discuss that from an approval perspective with the house provider, but technically with the basement provider.

I agree with Escroda’s scenario forecasts. With the slab on grade it will be simple, a knowledgeable district administrator will shake his head over the municipality and mentally apply "slab on grade" only to houses without a basement and replace it here with "OKKD."

More tricky is determining the reference point for the eaves height – where I assume the municipality bases it on the planned street. If that is your entrance side, fine. If you want to place the front door on the existing street side, it could come down to a roll of the dice between the building office manager and his deputy. Precisely because of such nonsense the determinacy requirement was "invented."

You will probably have to conduct the discussion with the house provider with a focus on approval, but technically with the basement provider.

So that means I have to bring both together at least for one appointment at a common table? Or into a conference call? Or just make sure they coordinate among themselves? Or how does that work?

The district administrator will shake his head over this through the municipality and mentally apply "foundation slab" only to houses without basements and replace it here with "OKKD".

He will do that, in case I would pursue legal action (which I still do not intend to do, purely theoretically). What does "OKKD" mean? Top edge of basement ceiling? If he replaces it like that, then everything is as it is now. At least for me. The city would then have a "clean" development plan, which however will no longer benefit anyone, since everything has been built. For that, I do not need to wake the district administrator.

Exactly because of such nonsense the requirement of definiteness was "invented"

And that was exactly what again?

The entrance will be on the existing side by the way. That is north. The living area faces south.

Ah, they really don't make it easy for you either...

: I will reply once more separately to your answer.

"Landrat" referred to the assumption that the ultimately responsible approval authority here is the district, which may be regulated differently depending on the federal state.

"Bestimmtheitsgebot" means that laws and regulations are not allowed to use vague terms. For example, "center of X" or "center of Y," but not "center of Kannstediraussuchen" or "center of Weißichselbernicht." For example, "the height of the manhole cover closest to the property on the entrance side" would be a clear height reference point. Or "top edge of the basement ceiling (or floor slab in non-basement buildings)" would be a "certain" term.

If a development plan specifies a knee wall Drempel, or the wall height also for a flat-roofed house eaves height, this does not harm the validity, however.