Questions zoning plan: distance carport-street, knee wall prohibition

Interestingly, at the beginning of the development plan's existence, there was apparently some validity gap (so the story goes), which is why NOTHING in the approximately 20 houses on that street section complies with the development plan.

I consider "some kind of validity gap" to be a myth. What is rare, but not impossible: a development plan may not be legally binding, yet many people build anyway, and in the end, only a handful of plots remain undeveloped, for which the torchlight procession to remedy the defective provisions is no longer worthwhile.

Hip roofs are cheerfully mixed with gable roofs with the wrong ridge direction, roof colors do not match,

That sounds like a development plan that never became legally binding. Then the building area is an "unplanned inner area," and consequently §34 applies – that is, the integration requirement and otherwise (of course excluding the Federal Building Code and the State Building Regulations) nothing else. In your case, this would have the following consequences:
Carport) in Rhineland-Palatinate to my knowledge no smaller distance to the street line than for a garage, at least for the posts as visibility-restricting components;
Roof) building height and roof pitch within the scope of the surroundings, roof shape free, knee wall not regulated separately.
Building mass) maximum number of full floors according to the surroundings, site coverage ratio and floor area ratio oriented to the actual average, not exceeding the actual maximum and especially a floor area ratio of 0.8.
Good luck!

"Irgendeine Gültigkeitslücke" I consider a myth. What is rare, but not impossible: a development plan may not be legally binding, yet many people still build, and in the end only a handful of plots remain undeveloped, for which the torchbearers of correcting the defective provisions no longer find it worthwhile.

Don't hold me to the exact wording... no one has been able to tell me how it really was so far, but what you say sounds (more) plausible.

Good luck!

Thanks. Next, I will ask the architect if he possibly knows or can find out (he lives in the neighboring town). Maybe the building authority can also help.

Maybe the building authority can also help.

... or your own look at the plan: there are a handful of fields with stamps and the mayor's signature next to a date ("Preparation," "Public display," etc.). If nothing is indicated under "Legal force," it did not become legally effective – if it did, it may have been challenged but then it would have had to be changed or withdrawn.

Correct. Compilation 1973, disclosure 1976, resolution 1977, legal validity 1979.

Correct. Established in 1973, disclosed in 1976, resolution in 1977, legal validity in 1979.

Yes, and - how many buildings that apparently do not comply with the regulations are from 1979 or newer?
How old is the existing building that you intend to demolish? - you could take a look at its building file...

The surrounding buildings are probably partially older… our current house dates from 1992 and also absolutely does not fit the plan, although it is fully within the scope of the mentioned development plan. The house to be demolished dates from the 1960s. The legal interplay between the very old houses, the younger and unsuitable ones, and the new houses to be built is not yet entirely clear to me. I will overlay the plan and Google Maps in the next few days so that the extent of the differences can be seen.