Municipal development plan insufficiently executed, what applies?

Of course they are allowed. A development plan only includes restrictions. It does not address common things that are allowed. I know a) the area, b) you can see on Maps what kind of existing structures are there.

It was exactly about the fact that dormers, cross-gables, and stuff like that are not allowed, right?

I understand the situation to be that they are not prohibited by the development plan, but merely unwelcome by the municipality (which in a constitutional state is just their irrelevant private opinion).

I understand the situation as that they are not unwanted because of the development plan, but merely by the community (which in a constitutional state is their irrelevant private opinion).

Yes, sorry. That’s what I actually meant. Thanks 11ant.

I understand the situation as follows: they are not unwanted because of the development plan, but merely by the municipality (which, in a state governed by the rule of law, is their irrelevant private opinion).

I understand it this way, that it is not fundamentally about the dormers, but about the knee wall, which is known to be higher than 50cm in the captain’s gable. So:

Gables, dormers or roof cut-outs are not mentioned in the development plan.

They don’t have to be. But that does not mean that they are not permitted.

The development plan merely stipulates a knee wall ("top edge of the shell construction... bottom edge of the roof surface") of 50cm. The knee wall is to be adhered to, that is clear. According to our understanding, however, the gables should not pose a problem under the Lower Saxony building regulations, as long as they are subordinate.

The OP or the municipality/district is concerned about the higher knee wall for the additional gables. Apparently, the district does not care, the municipality is calculating strictly.

I understand it to mean that it is not basically about the dormers, but about the knee wall, which is known to be higher than 50cm in the captain’s gable.

Oh, does it? - Captain’s gables have a higher knee wall than 50 cm only insofar as they are unusual in areas with vacuum cleaner knee walls. In general, they "adopt" the knee wall height of the main roof, provided it has a knee wall (or else they have a knee wall where the main roof has none). In the present case, I would never come up with the idea that someone does not apply a knee wall height limit to them. Of course, this maximum also applies here to the captain(s). The designation of a knee wall as a "Drempel" is common and is regularly regarded as a language variety in court, so the misnomer is not considered a lack of definiteness. The OP is welcome to clarify the discussion by showing the house pre-design used for the building inquiry :-)

Oh, does he?

Indeed. In most cases and usually also the captain's gables, which turn a one-story building into a two-story one.

The naming of a knee wall as a Drempel is common and is regularly regarded as a language variety in court,

Perhaps you should adapt to the OP and fellow discussants and also speak of the Drempel ;)

The OP is welcome to shed some light on the discussion by showing the preliminary house draft used for the building application :)

I think so too.