Floor plan optimization of a single-family house with a basement on a small plot

Who are you talking about: neighbors in this area who are also planning to build, or from next door who have already built?

One neighbor grew up in the village and still built her house on the parents' property, that was probably a few years ago. We look at her hedge from the property diagonally.
The other neighbor built 16 years ago. He also grew up in the village.
The two are again also direct neighbors themselves (as I said, northwest direction).

The notarial contract is not finished yet

But the neighbors have already signed. They have the same problem. Since you already maintain good neighborly contacts, you could ask them.

No, a central drainage system is not planned

Are any changes to the draft already finalized? The currently published justification for the development plan says: ... all properties are drained into a central drainage system within the public traffic areas.

a cistern is to be built for each property

I understood that as a possibility, not as an obligation. Private rainwater retention measures can still be maintained by the property owners.

with throttled discharge (0.5 l per s) of the collected rainwater into the "wastewater"

The rainwater channel is designed as a storage channel, from which the rainwater is discharged in a throttled manner into the neighboring receiving water 2 (parcels 181, 162, 191, 122, etc.), which is registered as a water body of the second order. "Wastewater" is somewhat misleading due to the apparently existing separation system. Since your property has a special location, I would ask whether you are allowed to discharge the rainwater directly into the receiving water. If the soil hardly absorbs water anyway, the difference to sealed surfaces is not that great.

No, a central drainage system is not planned, a cistern is to be built for each property (1 m3 per sealed 100 m2) with throttled discharge (0.5 l per s) of the collected rainwater into the "sewage".

Is the sewage then central or KKA? Can you also use the cistern for the garden or is it planned for firefighting water or the like?

I also find your suggestions great and that is how we currently imagine it too, piling up in the front (Sorry , I now know that you are a big fan of light ) and everything on one level, then via steps to the ground floor and down stairs to the back level as it is...

Oh, always these northerners...

But the neighbors have already signed. They have the same problem. Since you already maintain good neighborly contacts, you could ask them.

We will do that, we will meet them in person soon. I fear they are handling their plans a bit more casually, but maybe they know more details.

Have any changes to the draft been confirmed yet?

There is a drainage plan, have you found it yet? Otherwise, I would send it to you via private message, since you have already found the rest anyway. As far as I can see, there is a drainage ditch in the southeastern construction area near us, which then flows into the drainage ditch at our street (on plot 102/2, northeast of us). This drainage ditch near us then runs through the allotment garden area to the main street in the village (there is also another drainage ditch) and then eventually into a stream near the neighboring village. You can see this in the Geoportal Thuringia. Everyone from the southeastern area also drains into the drainage ditch near us, although the flow direction is indicated further into the village towards the west. I think this is meant by a central drainage system.

Is the wastewater then central or KKA?

Wastewater is central treatment plant.

Can you then use the cistern for the garden or is it planned for fire water or something similar?

It is not planned for fire water, can be used for irrigation.

Have you found it yet?

No.

Otherwise, I would send it to you via private message

Gladly.

I think this is meant by central drainage system.

According to the development plan information, I understood it differently. There is a wastewater and a stormwater sewer. The stormwater sewer discharges the rainwater at a controlled rate into the receiving water bodies. This also makes sense for the southern building plots so that the receiving water bodies are not overloaded during heavy rain. However, since the receiving water body is located between your property and the street, you would have to cross the ditch to discharge into the stormwater sewer.