ᐅ Plot for Construction Adjacent to Existing Building – "Single-Family House in Grandma’s Garden"

Hello housebuilding enthusiasts,

My wife and I are considering the possibility of building a house and are currently exploring our options. One suggestion came from my wife’s grandmother: "Build in my garden; there is plenty of space!" We would like to at least follow up on this idea by clarifying how feasible it is and what obstacles might arise. Since I have been reading this forum for some time, I would like to bring this question to the group:

How feasible is it to build an additional single-family house on the existing property, and what challenges do you see due to local conditions? (Topics such as the modalities of property subdivision / land division I would like to leave aside for now.)

Here are the basic conditions:

• The grandmother lives alone in a small house on the property with a double garage and a garden at the back. Her house should remain unchanged.
• The property is almost square, about 32m x 32m (about 1000 sq m / 0.25 acres)
• On the property: grandmother’s house is in the southwest corner, garage is southeast of center, garden/lawn is in the north and west
• Surroundings: low-traffic street to the south, terraced house with five units (and their gardens/terraces) to the east, single-family house with gardens to the north, multi-family house to the west (my parents-in-law’s house, 3 apartments/floors)
• In winter, the property has limited sunlight for several weeks due to a hill to the south casting a shadow
• We are in Baden-Württemberg, within town limits, with no specific development plan / zoning plan
• The desired house needs space for 2 adults, 2 children, and 2 home offices (she is a teacher, I work in IT and often from home)

To illustrate, here is the relevant excerpt from the land registry map with an aerial photo overlay (north is at the top):

In addition to the described conditions, there are the following two “wishes”:

• Preservation of the garden in the northern half of the property
• Retention of the double garage

As an interested layperson, and based on what I have read here about lighting conditions, orientation on the plot, etc., I consider #1 feasible by focusing the new building on the southeast part of the property. Regarding #2, I see a problem, however. To use the southeast area effectively for the new building, my understanding is that the garage would have to be removed. Or, conversely, if the garage stays, I only see a very narrow strip of land next to it on the right, and the problem that the garage would block sunlight from the southwest/west. Additionally, the garage would likely prevent a central subdivision of the property (vertically in the image).

Any lost storage/parking space would need to be compensated for at least partly.

How do you assess the described situation? I am grateful for any input and look forward to your feedback.

Best regards,
Sebastian