Single-family house with 160 m² - development plan, living area calculation

Hello everyone,

we are looking at a building plot where the development plan raises a few questions for me.

We would like to build approximately 150-160 m².
The desired plot is almost exactly 500 m². The "building volume" should be about 9 x 12 meters.

The development plan stipulates the following:
Number of full floors: I
Site coverage ratio: 0.4
Floor area ratio: 0.4
Eaves height: 4.5 meters
Ridge height: 7.0 meters
Roof type: GD
Roof pitch: 15-45 °

Federal state: NRW

What is not quite clear to me:
The floor area ratio means that we are theoretically allowed 200 m² of living space.
Do I understand correctly that due to the restriction "number of full floors = 1" the living space on the upper floor may be a maximum of 75% of the ground floor? To make it easier to calculate, for example 100 m² on the ground floor and 75 m² on the upper floor?
How important is the "clear height" here? How exactly are the 75% of the upper floor calculated with regard to the sloping ceilings?
With a knee wall between 1.0 and 2.0 meters, area is counted as 50% of the living area (please correct me if I misunderstood). What exactly does the "clear height," which must be 2.30 meters, mean regarding the calculation of living space?

How precisely can we plan here?

One consideration would be to make the rooms on the upper floor more "usable" by somehow building with dormers.

If I understand correctly, we also have leeway here due to the development plan.

Subordinate building parts (extensions, bay windows, cross gables) may exceed the maximum eaves height on a maximum of 2/3 of the building length in the designated WA 2 and WA 3 areas (see also regulations C.13 Roof structures / Roof cutouts).

What would you recommend or how would you build here?
From the floor plan perspective, we find the house "Auro" from "Kern-Haus" interesting. However, it says there that this would not be considered one full floor in NRW - I don’t understand why?

Important are 3 children’s rooms and an office.

Maybe you can shed some light on this.
So far, I haven’t been able to find anything here in the forum while searching.

Many thanks and best regards
Reviloo

I'm not an expert, but I also don't understand why the Auro has 2 full floors. 12m x 9m results in a 108sqm footprint. The upper floor is therefore allowed to have a maximum of 81sqm with a clear height of 2.3m. Here, the total living area of 70sqm is even well below that. That confuses me now as well

For knee walls between 1.0 and 2.0 meters, 50% of the area is counted towards the living space (please correct me if I misunderstood something). What exactly does the "clear height," which must be 2.30 meters, mean regarding the living space calculation?

Everything below one meter in height is fully excluded from the calculation, and between one and two meters in height the area counts as half – whether there are actually knee walls, dwarf walls, or nothing along these virtual lines is irrelevant. Clear height means how tall a person can be standing on the finished floor without hitting their head on the ceiling. The height of 2.30 m is not relevant for the living space calculation itself, but rather for the question of full floors and their area calculation. That means all areas under 2.30 m do not count, and above that count fully. Since the areas mentioned on the website for the Auro (70 sqm attic to 87 sqm ground floor) relate to the living space calculation (i.e. with the 2.0-meter relevance) and not to the full floor-relevant 2.3-meter areas, I assume that the Auro meets the single-story requirement and the statement that it does not is based on the incorrect assumption that 70:87 are already the relevant comparable areas here (80% would of course exceed 75%). However, the Auro has a 75 cm knee wall and an apparently observed one-third restriction of the dormers. The house provider operates nationwide, including in federal states with a 66.66% full floor limit, and is certainly familiar with the issue. I therefore assume that it should be possible not only in NRW but even with somewhat higher knee walls in the 75% federal states. Otherwise, with the study dormer there would still be a saving option; I do not see the dressing dormer as dispensable because of the stair headroom. Introduce yourself here with the completed questionnaire from the top of the floor plan section, then more concrete alternatives can be discussed – in the form of the Auro presented on the website with a 75 cm knee wall I consider it possible until proven otherwise. That is why the statement "Number of full floors: 2 in all federal states except Saarland" also confuses me.

For knee walls between 1.0 and 2.0 meters, 50% of the area is counted as living space (please correct me if I misunderstood something).

Anything below one meter in height is completely excluded from the calculation, and between one and two meters in height, the area counts half.

No, that applies to living space calculations in, for example, residential construction. And I think your explanation is somewhat confused and therefore flawed. And honestly: it is very confusing to read.

The floor area ratio only refers to full floors.

To avoid a full floor classification:
The 75% refers to the clear room height of 2.30 (assuming your numbers apply to your federal state – other federal states use 2/3 of the area or a clear room height of 2.20 (?)).
This concerns the definition of a full floor. Only the area with a room height of 2.30 counts; the rest does not.

So you could build 200 sqm including the terrace. Then be careful with the attic not to reach full floor status:
Balance it with the knee wall and roof pitch. So if walls were just lines, you could build 350 sqm.
But you don't want that. If you take your 9 x 12 and plan with a 150 cm knee wall... just take a graph paper and draw the section. Then you can quickly see how your little house could look if you comply with the 75% area.

Roof type: GD

What kind of roof is that?

The Auro gets its two-story character through its two gables combined with a steep roof and knee wall.

What ypg wrote about the ²

The state building code states the following: “Full floors are above-ground floors that have a clear height of at least 2.30 m. A floor is only considered a full floor if it has the height mentioned in sentence 1 over more than three quarters of the floor area of the floor below.”

No, that applies to the calculation of living space in, for example, residential construction. And I think your explanation is also somewhat tangled

Correct, and I did not claim otherwise: I only addressed the 1.0 and 2.0 meters because they appeared in the OP’s question, and they are only relevant for living space calculation according to the Wohnflächenverordnung or DIN – however, the living space figures given in the brochure of the mentioned house model are stated, which I considered responsible for the negative assessment. So once again: non-inclusion and partial inclusion in regard to one or the other living space standard only matter there; for the full storyy, the attic’s 2.3 m clear height is decisive, respectively in the basement 1.4 m average height above the original terrain. Areas with less than 2.3 m clear height in the attic are therefore not relevant – except in Berlin and Brandenburg, which only recognize full stories, provided there are rooms for occupancy.

What kind of roof is this? The Auro gets its two-story character through its two gables in combination with a steep roof and knee wall.

GD probably means here collectively the category of all pitched roofs. Why the Auro is supposed to be two-story with only 75 cm knee wall according to the provider’s information, except in Saarland, I have not yet understood. However, I think if at all, then so narrowly that it could be offset again by a bay window on the ground floor. Or one takes a consistently low knee wall of 50 cm (or partially bricks up the knee wall). So if it absolutely wants to be the Auro, there would probably be ways and means. In any case, I do not see it far from a possible house form here, so it may serve the OP as a conceptual aid for what would be possible on their plot.