First steps house construction, appointment at the building authority

We have an appointment at the building authority on Tuesday. Since we are still newcomers in this field, maybe you could help us with which questions we definitely should/must ask. We would be very grateful for that.

Good day to everyone.
We, 33 & 35, no children yet, plan to build a house in the postal code area 63150 Heusenstamm. But we already think about the future, that we naturally want children (max 2) and have to plan for that.
My girlfriend’s parents own a house with a very large plot of land. And on the property there is currently a garden. We plan to build there. It’s about 2 terraced houses, since her brother also wants to build with us but we want separate houses/entrances etc., so everyone for themselves.
Therefore, we have already decided to build 2 terraced houses/semi-detached houses each for ourselves with a basement.

Now we want to get more detailed information about what the first steps are.
Who do you have to contact first who will look at it and tell us how wide and how high we are allowed to build, what we all have to consider, etc.

Therefore, we would be very happy if you could help us with the first steps on how to proceed now and whom we should contact first.

We thank you very much.

Basically, it is important to know whether the property is buildable at all.

Is building in the second row allowed?
The garden property: is it building land?
Where is the building envelope?
Does the building envelope offer space for an (additional) semi-detached house?
Can the property be divided in reality?
Does it have to be divided in reality?
What does the floor area ratio look like? Are there still capacities for further development?
How much floor area has already been built over with the old house?
How is the rear property accessed? Infrastructure?
What does the development plan say about the construction?
Is a semi-detached house or only a single-family house allowed to be built?
How many parking spaces have to be built per unit?

Example 1:
Property of 25 x 60 meters (1500 sqm), the building envelope starts at 5 meters and is 35 meters deep. The existing house is located at the building boundary and has a depth of 12 meters, behind it a 3 meter terrace, then there must be a 3 meter distance to the property boundary. The building envelope then still offers 17 meters in depth for construction, of which 3 meters fall away to the boundary of the front remaining property. That would work.
The existing house is left with 23 meters or 506 sqm minus 3 meters access path.

But if the development plan provides a floor area ratio of 0.2 and the existing house has 180 sqm, then 180 sqm for 506 sqm property is far too much.
That would be an important question to clarify if your situation is similar.

Draw out what you know and sketch it so that you can adjust to your situation.

Example2 You agree on an ideal division because of this problem, the property will be "divided" after 30 meters. We now assume that there is no building boundary at the back. Due to the already built 180 sqm in the existing structure, the new building is left with 120 sqm of buildable area at a floor area ratio of 0.2. That is not enough for an adequate semi-detached house at the front and back, just about 100 sqm of living space on the ground floor. Possibly, a two-family house could be built, with one family on the ground floor and the other on the upper floor...