What are the first steps in building a house?

Good day to everyone.
We are planning to build a house.
The parents of my girlfriend own their own house with a very large plot of land. And on the plot there is currently a garden. We plan to build there. It concerns 2 terraced houses, as the son 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 with basements.

Now we wanted to get more detailed information about the first steps.
Who do you have to consult first, the person who looks at it and tells us how wide and how high we are allowed to build, what we have to consider, etc.?
I heard that you have to go to the municipality regarding the type of construction. Is that true?
Is it true that there has to be a minimum distance to the neighbor? If so, how far must it be?
If possible, we wanted to find the companies such as electrician, roofer, plasterer, structural builder etc. ourselves, instead of having everything done by the architect, for example...

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.

A planner who knows what he is doing, understands the development plan, and advises you concretely.

Hello BeHaELja Planer? So an architect? Thanks for the information

Does not have to be a freelance architect... but you are asking questions that cannot be answered from a distance without knowing the [Bebauungsplan]. There are both specifications regarding the type of development and the distance to the neighbor - both anchored in the [Bebauungsplan] or in the law. You also call it a townhouse but actually mean a semi-detached house (?). Furthermore, you will need a cost estimate for your loan... none of these are topics that a layperson can just whip up - hence the somewhat cynically sounding comment about the planner.

I would first check the development plan (Bebauungsplan) to see if further development of the property with a separate house is generally permitted. If yes, one must verify to what extent the floor area ratio/site occupancy index is exhausted. Where are the building limits? Is the rear part of the property possibly just garden land and thus not building land? In single-family house areas, an open construction method is often mandatory – by this I mean only single-family houses are allowed, no terraced or semi-detached houses. It is not uncommon for older houses to be on larger plots, but the development plan also does not provide any further possibilities for additional development. If you have questions, you can use the office hours of the building authority and ask your questions there. They can also say whether it is worth submitting a building application against the development plan.

In single-family house areas, open construction is often mandatory - by that I mean, only single-family houses may be built, no terraced or semi-detached houses.

Unfortunately, that is not correct. Open construction means that buildings must be built with a side boundary distance; regardless of the house type. This is defined by the development plan and textual specification. Rhine greetings