Thanks already for your input and food for thought.
Regarding the questions: The granny flat was originally not planned. During the planning process, we commissioned a survey to avoid surprises later on, which was a good decision. The result was that the terrain in the northeast is almost 2 meters lower than the southwest street level. In the lower area, a large double garage is planned. This led to the consideration of investing 30,000 euros in gravel and foundations or 100,000 euros for a second apartment. However, the additional investment is offset by tax relief, rental income, additional KfW benefits, etc. Later, the apartment can be used for parents, children, or as a holiday apartment (we live in a holiday area)...
The construction volume is currently estimated to be around 550,000 to 600,000 euros. Our targeted rate is 1,300 to 1,400 euros plus a share from the rental income.