House Construction: Step-by-Step Guide

Garden land in the property tax assessment does not exclude building land, these are two different things. From the description, I suspect a building gap in a §34 area. Upload a cadastral map, an aerial photo, or something similar, then one can speculate less vaguely. Look up the search term "procedure" here in the forum, your question is almost an FAQ.

If I could start over again, I would proceed roughly as follows:

1. Find the development plan
2. Check the budget with the bank non-bindingly. On this occasion, also make a list of your monthly expenses. It will grow for several weeks until everything is included. Honestly assess possible own contributions
3. Clarify your own wishes. For now, ignore buzzwords as much as possible
4. Look for inspiration for 3., for example in the floor plan subforum and in the catalogs of [Typenhäuser]
5. Read along here in the [Baukostenforum]
6. Obtain literature on how to actually read a building specification and what to pay attention to
7. Consider architect or [Typenhaus]
8. Only now talk to a salesperson. As long as you don't know what you want, there is no point in discussing any house models and prices

There is no development plan available when you request it.
So it seems there are no regulations.

That doesn’t have to mean anything. Ask the building authority. Most likely then according to Par. 43, and that’s about area. Then it doesn’t look so rosy for the development. Upload a screenshot of the surroundings with a marker on the property.

Where does one start?

There:

This is still garden land and must be converted into building land.

I would first thoroughly deal with the topic of construction costs and do a lot of research in this sub-category! You will have enough time for that at first.