Plot of land from the neighbor built over by 20m².

The plot is 780m2

So the wall stands about 50cm on your colleague’s property? How thick is the wall? How high is the wall? Is it clear whether soil was dug out or filled in? Which property is higher?

that is being divided to build 2 semi-detached houses.

Your colleague bought the entire 780m² two years ago and now wants to divide it roughly in half?

And “bought as seen” does not exist in case law, so why for a plot of land?

That depends on how the plot was described and shown. If your colleague did not have the boundary line shown during the inspection, then that is his problem if he plans only based on the cadastral map without taking any measurements on site. For a solid retaining wall, it is hardly a hidden defect.

What facts would be needed to evaluate this better?

Is there a development plan? Was the wall subject to approval? If yes, is there an approval? Was a structural stability certificate required? Is it available? Was the filling/excavation subject to approval? If yes, is there an approval?

A plan of the property was given at the time of purchase

But probably only an excerpt from the cadastral map and not a surveyor’s site plan with topography (heights, slopes, walls, fences, protected trees…).

At least the planning is useless.

Since this is a bit like a game of telephone (facts, colleague’s story->tomtom79, tomtom79->forum), the exact sequence is hardly reconstructible here. It could be that the seller deliberately deceived, or that your colleague was too careless.

But is a letter really enough to move a boundary?

No. The boundary has not been moved. Your colleague owns a plot with a retaining wall. If he is unlucky, he is also responsible for its maintenance. If the neighbor admitted to having built the wall, your colleague should urgently get that in writing.

That the seller did not pass on this letter should definitely hold him liable.

No. If all parties pretend not to know and no documents surface, the wall, as a fixed structure, is part of the property and belongs with all rights and obligations to your colleague.

well if the garage is supposed to go there then just inform and tear down the wall. Attach the garage to the rest of the wall. it will create a seamless look. But everything after consultation.

I don't understand the whole thread...

They're all related anyway - have they just talked to each other?
Or is this supposed to be a guessing thread...?

Neighbor and seller are related, the buyer/colleague probably not. As I understood it.

I would also first want to see planning documents and/or pictures. 20m2 is quite a large part of the garage.

If the planner has already drawn up plans without surveying the property, the planner is responsible for the additional costs. That is part of the basic investigation.