What about contaminated sites on the properties?
None. On the contrary, the soils are excellent, as the justification for the development plan states:
As a further (at least partial) compensation option, soil improvement in the form of topsoil application was also examined. However, this type of compensation is not considered for soils whose soil function "natural soil fertility" corresponds to an evaluation class of 3 or 4. The soils around A-city mostly correspond at least to evaluation level 3 regarding the soil function "natural soil fertility" and are therefore hardly upgradable.
You might have been a bit suspicious with "every single detached house owner may do whatever they want on their plot, as long as they are the first."
And what conclusion would you have drawn from that as a "cheap" land seeker?
The planning is scandalous and the way the builders are treated makes it worse. But who has the time, money, and desire to litigate with the municipality where they want to continue living for years?
since the development works are fully on schedule
If there is a schedule, there must also be an execution plan. Has no one requested it?
Is the embankment of the road construction body on your property?
Was there already an embankment built up on your property at the time of purchase?
Is there a clause in the purchase contract regarding toleration of the embankment?
Aside from the fact that I see no reference to the content in the specified legal basis (§ 74 para. 2 no. 3 State Building Code) for §5 of the local building regulations, which is ambiguous and indefinite, it alone does not justify the use of your property (cf. Federal Administrative Court 4 CN 5.08). Perhaps a starting point for a specialist lawyer, or at least to strengthen the negotiation position.