Eaves height questions - property is subject to development plan

The consultant is right.

First: say goodbye to the 75%. They result from an interplay of roof pitch, knee wall, story height, and gable length (the further the eaves sides move apart, the larger the roof triangle becomes). However, all the values are more or less already defined in the development plan, making it difficult to have 75% of the upper floor without a slope. Unless you build with a cross house/cross gable. I estimate that with a max. 33° roof pitch and the given eaves heights, you end up somewhere around 60%.

One should perhaps also keep the background of the B-Plan in mind: it ensures that the houses nestle closely to the slope and thus all plots get good light and visibility conditions. Pushing the limits: ok! But 2 full stories are not wanted for good reason.

Sorry for this probably very simple question.

As you can tell from the answers, this is anything but a simple question.

I realize that I have shone with ignorance

By no means. You have exactly uncovered the weakness of the development plan when a gable roof is not planned.

However, if I understand the eave height correctly, it does not refer to the attic/top floor/setback floor, correct?

That is not correct. The eave height for the setback floor in the case of a flat roof is the height of the parapet, for a hipped roof according to the development plan definition it is the height of the upper edge of the rafters in line with the exterior wall, each measured down to the natural ground level. With the recessed setback floor, this point falls inside the building, so the eave height cannot be complied with on the uphill side. Your neighbor has probably applied for an exemption. With a good justification, that should also be possible for you. Just ask the building authority after consulting with your neighbor how the requirements are interpreted there and what options exist.

a bit unspecific

A sectional drawing of your project analogous to the system sketch in the development plan would help to avoid misunderstandings.