Property available, development plan changed - solid house?

Development plan old: 2 full floors, development plan new: 1 full floor. This will not have been done without reason (separate document "Begründung zum Bebauungsplan xxx", which may be attached to or referred to in the text portion of the development plan).

Correct.

Then the development plan would not have been changed at all. Why should exceptions be approved in that case?

The reasons can be varied. Usually, one wants to address a specific problem. However, this may not apply to the specific neighboring properties, so that there is little against an exception in that case. That is, in my view, the chances depend on how much the council and administration like to insist on principles. Asking costs less than re-planning ;-)

So, a little update: The plan is actually unchangeable. I have an appointment with a prefab house manufacturer on Tuesday and am curious about the production and assembly time. I will report back with the results then. By the way, the solid construction builder mentioned a period of 7 months yesterday (really fast too).

Best regards

...and I am curious about the manufacturing and assembly time.

Much more relevant nowadays, given the capacity utilization, is the waiting time for the actual start of construction, the "slot" to build.

By the way, the solid builder mentioned a period of 7 months yesterday (really fast too).

Yes, of course. The actual construction usually doesn't take longer, unless it's an architect-designed house with less efficient logistics than with a general contractor. A prefab house is assembled in 3 days, followed by interior work over 3 months. But: the start takes time. For a solid house, half a year to 1 year, and for a prefab house from well-known house building companies often over a year. Only with small companies can you hope not to be just one of many and have to wait in line.