Is reserved land reasonably buildable?

Would that depend on how far the basement protrudes, correct?

No. Actually, the full story status is clearly defined in every federal state. That also applies to basements. To be able to assess the neighboring development area and the 0.8, one would need to know more about the terrain. Certainly, you had some logic, even if it is not immediately apparent. Then I am curious about how it may be for you.

North slope. What are the dimensions of the building plot? Is the garage allowed on the boundary?

North slope is not really the best choice right now. The building plot measures approximately 28m x 15m. Garages must not be more than 6m away from the property boundary to the access road. I couldn't find anything else in the development plan. Nor in the state building regulations.

No. Actually, the full storey height is clearly defined in every federal state. This also applies to basements. To be able to assess the neighboring building area and the 0.8, you would have to know more about the terrain. Surely you had some logic, even if it is not immediately apparent. Then I am curious how it will be allowed in your area.

Ok, according to the state building code this: (5) Storeys are above-ground storeys if their ceiling top edges protrude on average more than 1.4 m above the ground surface; otherwise, they are basement storeys. Voids between the top ceiling and the roof covering, in which habitable rooms are not possible, are not storeys.

(6) Full storeys are storeys which protrude more than 1.4 m above the average measured ground surface and, measured from the top edge of the floor to the top edge of the floor of the storey above or to the top edge of the roof covering of the roof above, are at least 2.3 m high.

That means less than 1.4 m protrusion above the average measured ground surface = no full storey.

Ok, according to the state building code, it says: (5) Stories are above-ground stories if their ceiling edges protrude on average more than 1.4 m above the ground surface; otherwise, they are basement stories. Voids between the top ceiling and the roofing, in which habitable rooms are not possible, are not considered stories.

(6) Full stories are stories that protrude more than 1.4 m above the ground surface measured on average and are at least 2.3 m high, measured from the top edge of the floor to the top edge of the floor of the story above or to the top edge of the roof covering of the roof above.

That means less than 1.4 m protruding above the average ground surface = not a full story.

That means less than 1.4 m protruding above the average ground surface = not a full story.

It is also important which ground surface is taken here. In NRW, this is, for example, to my knowledge, the original ground surface. In our area and almost all neighboring areas, the basement was dug out on the side to allow for more windows. As a result, in the finished houses, the ceiling is more than 1.4 m above the new ground surface. Fun fact, underground more boundary construction is also possible. We have a 12 m long garage, of which only 8 m is above the original ground surface. That made it possible to classify it as a boundary garage to the neighbor’s garage.

In NRW, you are also allowed to build underground outside of the building envelope. At least that is my understanding of the building code.

The "elevation of structural facilities" or an indication of the basis of the established ground floor floor height cannot be taken from the development plan. That would depend on how far the basement protrudes, correct?

Basically understood correctly. However, forget my statement

The slope runs roughly diagonally, so that a square floor plan or any house axis positioned lengthwise or crosswise would be the same.

insofar as I used the term "slope." But here there is a rise from the street side Rosmarinstr. to the garden side towards Auf der Ay. The plot lies uphill from the street and the basement faces the street. Trying to utilize the height of a non-full-storey basement would therefore be rather unattractive, and you can basically forget this aspect. By the way, the state building code applies in the version valid at the time of the development plan. I wouldn’t want to greatly increase a basement here, but rather only let a cellar protrude towards the street as a plinth as far as necessary. Seek your salvation in a one-storey building.

How is this handled if this information is missing? Doesn’t it always have to be defined?

This is often left unregulated as far as reasonable in older development plans. Only in the course of abusive exploitation were setback rules introduced and the development plans became increasingly restrictive.

For the neighboring development plan, at least "For new buildings, the ground floor floor height must not exceed 0.8 m above the carriageway of the development road as measured on average along the street layout." Even if this doesn’t really help here.

Regulations of newer development plans help insofar as it is recognizable against which excesses the plans have become more resilient and the building authorities simultaneously more petty. So, probabilities of exemptions can be estimated. But you are probably talking about the (similarly old or only slightly newer) applicability area on Rosmarinstraße at even house numbers (?), whose regulations you would then have to read with a different perspective, as they refer to the valley-side conditions. So: from these viewpoints, I see a clear longitudinal axis (eaves, ridge) parallel to the street, shallow building depth, and a moderate knee wall of probably effectively around 80 cm or less. And no, you definitely don’t have to worry about that.

It is also important which ground surface is used here. In NRW, this is, for example, to my knowledge the original ground surface. Here and in almost all neighboring areas, the basement was excavated laterally to allow for more windows. As a result, in the finished houses the ceiling is more than 1.4m above the new ground surface. Fun fact, more boundary construction is also possible underground. We have a 12m long garage, of which only 8m is above the original ground surface. This made it possible to have it as a boundary garage to the neighbor’s garage.

In NRW, underground construction is also allowed outside the building window. At least, that is my understanding of the building regulations.

The building authority says the following, as has practically already been answered here: For the requested plot, the single-family house is not separately defined. The single-family house ultimately results from the terrain and the permissible number of floors. The basement may not, in this case, be a full floor according to calculations. The State Building Regulations 1983 apply here; the upper edge of the basement ceiling may not protrude more than 1.4 m above the average measured ground. From this arises a possible area for the single-family house.

That means the possibility of a "partially exposed" basement with larger windows facing north/northwest would exist. I have looked at this in an example. If the building body is placed more or less in the middle of the building window to accommodate the slope and the length is about 11m, the following could result:

(Original) higher level of the building corners (whether this corresponds to the State Building Regulations 1983, no idea): 551m, 552m, 551.5m, 552.5m → approx. 552m on average (if the entire parcel is taken as a basis, a similar value results). That means the upper edge of the basement ceiling (then also roughly the single-family house?) may be at 552m + 1.4m = approx. 553.5m. Have I understood this correctly? This means that one would practically end up on the highest area of the property in the south and the basement would lie approximately at the level of the terrain in the north at 550.5m.

Problem: Not really attractive due to the hillside location. That makes sense. I will still try to sketch this in a section.