A real two-family house with similarly large living space

Maximum heights/limits: Plinth height max. 0.80 m; eaves heights max. 4.30 m; ridge heights max. 8.80 m to the nearest manhole cover (which is approximately standard curb depth, so about 20 cm?)
further specifications: tree planting on the property, initially not relevant for the development

Requirements of the builders
Style, roof shape, building type: none, preferably mansard roof for better use of the attic
Basement, floors: fully developed and usable basement + 2 full floors + attic

Eaves height 4.30 m does not correspond to 2 full floors. 2*3m equals 6 m
Ridge height 8.80 m does not make 3 stories 3*3 m 9 m. That doesn’t even include the roof slope.

Here, a 1.5-story house as a duplex with about 1.3 m knee wall fits. You can look for something like that in detached houses.

Your planning can go in the trash.

Talk to the bank again about a higher loan.

Do you already have the plot?

What does the development plan say about roof structures, are semi-detached houses allowed and boundary constructions with garages or carports?

Again about the budget. Depending on the construction site, you can expect 2000 - 2500 euros per sqm for standard equipment without additional construction costs, outdoor facilities, garage or carport, and interior fittings such as the kitchen. 2000 euros will no longer be achievable in most parts of Germany without massive personal effort and friendly trades. Once you actually start building, quite some time will pass and it will not get cheaper.
In your cost breakdown, for example, the basement is massively undervalued. It should at least partially be developed as living space, it is completely within the thermal envelope, so why should it be so cheap? There are good reasons why nowadays houses are often built without basements.

I estimate the costs without knowledge of the construction site and personal contributions and for around 350 sqm (I haven’t exactly calculated how many sqm the design has) at:

2000 x 350 = 700k
Additional construction costs including excavation 50-60k
Outdoor facilities 30k
Garage/Carport 10-30k (depending on what it is)
Kitchen/interior fittings at least 15k

In the end, these are just 2 stacked houses, so it only costs somewhat less than 2 semi-detached houses.

Read up on the topic of construction material price developments. There is now an article about it in almost every daily newspaper.

This could be what a fair division looks like.

Thank you for your feedback.
The development plan cannot be linked. It is 27a and area W2 of Teltow (Brandenburg).

Why does a development plan allow 2 full floors if this is never feasible with the eaves height?
Semi-detached houses are not permitted on one plot, only one half each. Statement from the building authority: "A two-family house is possible with a shared entrance." We will revise our planning again.

I will ignore the personally directed statements then. Welcome to the world, there are also inexperienced people who write in a forum to ask for help.

Welcome to the world, there are also inexperienced people who write in a forum to ask for help.

The inexperienced person should then also read the statements about the impracticality and not overlook them. Before the inexperienced person improves the advice of the advisor and does not believe it, they should first think and possibly ask. Ignoring facts no longer works on page 3 in a forum... in real life, you would have a construction ruin in the shell stage.