Development plan of 1998 - Setting the eaves height to a maximum of 3.00m

Thickness plays a minor role in the eaves height, after all, it’s about the interface.

But of course, for example, with a 35° roof pitch, 10 cm thickness of external rafter insulation also increases the eaves height by 7 cm. So I find the argument quite understandable. Together with today’s thicker floor structure in the attic (the one of the ground floor is neutral in this case), that adds up.

1.5 Number of storeys

There are no half storeys in building law, so they cannot be prescribed either. In your case, single-storey construction is stipulated.

Gable roof is prescribed

That is also incorrect. You can already see this from your neighbors, some of whom have built half-hipped roofs. Only the roof pitch is prescribed. I personally do not like shed roofs and we do not know your planning, but here too there could be an approach to a solution, since the upper edge of a shed roof counts as the ridge, the height of which is not limited in your development plan.

The eaves height is not further defined.

However, I have reread two rulings of the OVG NW dated May 3, 2010, file no. 7 A 1942/08 and November 6, 2013, file no. 7 D 16/12.NE. Based on these, I would like to correct my term "contestable" in #2 to "interpretable." Since the lower reference point can be determined well according to the definition in the development plan, it is only about the upper one, which according to the first-mentioned ruling “is to be set at the lower edge of the relevant roof outer surface.”

I would like to avoid lengthy discussions and proceed as pragmatically as possible.

A challenge to the development plan would certainly not be effective and—without knowing the reasoning behind the development plan—probably also hopeless here based on the above considerations.
However, the court’s view on the eaves height already gives you the desired independence from the roof structure, since, unlike in the calculation of setback distances, reference is made to the lower edge of the roof. Whether this will be enough for the—obviously not only by me regarded as pointless—3 m clear height, you will then have to check with the exact street and plot heights.

However, the court's interpretation of the eaves height already grants you the desired independence from the roof structure, since, unlike in the calculation of distance areas, reference is made to the underside of the roof.

So basically "underside of rafters"? - then an insulation above the rafters would be neutral and not an exceptional argument.

So basically "bottom edge of the rafters"?

Well, that’s exactly the question. I’ve read the ruling again and come to the conclusion that it describes the eaves just as vaguely as other planning law discussions on the topic of eaves, namely not at all. I am now more inclined to think that the building regulations view is represented here, although formulations like "height of the roof connection," "lower edge of the relevant roof exterior surface," and "this intersection limits the pitched roof surface of the chosen roof construction downwards" confuse me. What is certain: according to building regulations (i.e., for the calculation of setback distances), the locally usually unrecognizable intersection line of the exterior wall with the upper delimiting surface of the roof is meant. It is also certain that the building regulations view does not have to agree with the planning law view, which is why the development plan must provide clarity. Furthermore, it is certain that the development plan does not define the term "eaves," which is definitely a deficiency but also gives the approving authorities the option to proceed without exemption from the point most favorable to the builder. Three meters referring to the setback-relevant "eaves point" above the street — so not even above the finished floor level — seems very tight to me in any case.

Just asking by the way: no matter where and how exactly the eaves are measured, he will still never reach the 3m ceiling height with this development plan, right?

he will never reach the 3m ceiling height with the development plan, right?

Yes, because the 3m refers to the street level and not to his own floor, whose height is not fixed in the development plan. Therefore, he can lower the floor as much as he wants.