ᐅ First Steps in Building a House, Appointment at the Building Authority (Building Permit / Planning Permission)

We have an appointment at the building authority on Tuesday. Since we are still beginners in this area, could you possibly help us with which questions we should definitely ask.

We would be very grateful for that.

Good day to everyone.
We are 33 and 35 years old, have no children yet, and plan to build a house in the postal code area 63150 Heusenstamm. However, we are already thinking about the future and want to plan for children as well (maximum 2).
My girlfriend’s parents own a house with a very large plot of land. So far, there is just a garden on the property. We intend to build there. It involves two terraced houses, as her brother also wants to build, but we want separate houses/entrances, so each has their own.
Therefore, we have already decided to build two terraced houses/semi-detached houses, each for ourselves, with a basement.

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

We would be very happy if you could help us with the initial steps on how to proceed and who we should contact first.

Thank you very much.

Fundamentally, it is important to determine whether the plot is buildable at all.

Is building allowed in the second row?
Is the garden plot designated as building land?
Where is the building zone (building envelope)?
Does the building zone allow space for an (additional) duplex?
Can the plot be legally subdivided?
Does it have to be legally subdivided?
What is the floor area ratio (FAR)? Are there capacities for further development?
How much ground area has already been built over by the existing house?
How is the rear plot accessed? Infrastructure access?
What does the zoning plan say about development?
Is it permitted to build a duplex or only a single-family home?
How many parking spaces must be provided per unit?

Example 1:
Plot measuring 25 x 60 meters (82 x 197 feet), building zone starts at 5 meters (16 feet) and is 35 meters (115 feet) deep. The existing house is located at the building boundary and measures 12 meters (39 feet) deep, followed by a 3-meter (10 feet) terrace, then a 3-meter (10 feet) setback to the property line is required. This leaves 17 meters (56 feet) depth available for construction within the building zone, minus 3 meters (10 feet) for the boundary of the front remaining plot. This layout would work.
The existing house would have, after deducting a 3-meter (10 feet) access path, 23 meters (75 feet) and 506 square meters (5445 square feet).

However, if the zoning plan specifies a floor area ratio of 0.2, and the existing house covers 180 square meters (1937 square feet), this would be far too large for the 506 square meter (5445 square feet) plot.
This is an important question to clarify if your situation is similar.

Draw out what you know and sketch it to better understand your situation.

Example 2
You agree on a conceptual division regarding this issue, and the plot is "divided" after 30 meters (98 feet).
We will assume that there is no building line at the rear.
Due to the existing 180 square meters (1,938 square feet) already built, the new structure has 120 square meters (1,292 square feet) of buildable area left with a floor area ratio of 0.2. This is not sufficient for an adequate semi-detached house at the front and back, offering only about 100 square meters (1,076 square feet) of living space on the ground floor.
It might be possible to build a two-family house here, with one unit on the ground floor and the other on the upper floor...