Widen the building plot by 3 meters

Then the land consolidation has already taken place. It was a large field and is now several small building plots distributed along a new road.

The motivation for enlarging the property is to have more distance between my house and the boundary to the left neighboring property. So instead of 3m, the distance would be 6m.
But now I have learned that I will not achieve this through the property enlargement. The following reason was given to me by a family member:
According to the development plan, there is a buildable area. This extends across the full width of the property (and also extends over the neighboring properties). I thought I was free to place my house anywhere within this buildable area. But apparently, according to building law, it must be placed to one side. That means a distance of 3m either to the left or right property boundary. I am already planning a garage on the right. So I have to keep exactly 3m distance to the left. 6m distance would not be allowed?!
Can someone confirm this?

On the right, I am already planning a garage. So I have to keep exactly 3m distance on the left. A 6m distance would not be allowed there?! Can someone confirm this?

No. 6 m is also possible.

Ok, thank you. The building boundary and building lines were probably confused. According to the development plan, there is a building boundary but no building lines. From this, I conclude that I can place the house freely within the building boundaries and considering the distances to the street and neighboring properties. I will therefore continue to pursue my project and hope to be able to buy 3m more from the neighboring property to have more distance from the neighbor's future garage or house.

According to the development plan, there is a building boundary but no building lines. From this, I conclude that I can place the house freely within the building boundaries, taking into account the distances to the street and neighboring properties.

That is correct,

I thought I was free to place my house anywhere within this developable area. But apparently, according to building law, you have to place it on one side. That means a distance of 3m either to the left or right property boundary.

This is certainly a misunderstanding. It is not stated like that in any law. It could theoretically be stated in a development plan, but that would be nonsense. There are cases where it says that building on one side is required. But where distances should be, I only know them as minimum distances. The building line can be seen as an exception, but it usually only occurs on the street side, i.e. only at corner plots then on two sides as well. If you have to build on one side, that applies to the main building. I can only imagine that in a completely newly developed building area in the case of semi-detached houses. But that is rather rarely mandatory ("D"), usually it is optionally allowed ("E/D" or "ED" = single or semi-detached houses).

According to the development plan, there is a developable area. This extends across the full width of the plot (and also extends over the neighboring plots).

What you mean by that is the building envelope. This is a "frame" drawn over several plots, which typically results from four definitions: on the left from the lateral boundary distance (building setback) of the leftmost plot, which are captured in the frame; on the right similarly from the rightmost plot; on the street side from the desired distance to the street; and on the garden side from the desired maximum building depth, measured from the street-side distance.
In the drawing, this envelope is continuous, but it only actually means zero side boundary distance in the case of semi-detached houses. The colors of the lines that define this frame have a meaning: blue are building limits, beyond which you cannot build; red are building lines, which you must build exactly on. Building lines only occur on street-side boundaries of building envelopes, and usually only where the development is meant to have an avenue-like character – therefore rather rare in building areas with cul-de-sacs.
For single-family housing or semi-detached houses on the free side, the boundary distance results purely from the textual regulation (if the development plan has no special wishes, then actually only from the law, i.e. the state building code). Only for the "last" / "end" plots is the lateral distance also shown in the drawn envelope. But even for these plots, some buildings (a typical example: garages up to a certain size and without the attachment of living rooms, etc.) enjoy a "distance privilege," which varies depending on the state building code regarding lengths and roof heights.

So I will continue to pursue my project and hope to be able to buy 3m from the neighboring plot to have more distance to the neighbor’s future garage or house.

What is your motivation for this – would that be the nicest patio side in terms of cardinal direction? What absolute numbers are we talking about: plot width without changes / desired house width?

Show the site plan of the area with the building envelopes