Building a retaining wall in the garden: formwork stones or L-stones?

The best thing is to coordinate with the neighbor. If you both deviate significantly from the original terrain, it also makes sense to share the costs and only reinforce once.

We deviate approximately plus 1m from the natural terrain, the neighbor about minus 2m...

I imagine it’s difficult since the neighbor has dug away steeply right from the property boundary and has not secured it. I fear that I won’t get around needing a concrete foundation even with L-shaped stones.

Then it doesn’t have to be you placing the stones, but the neighbor.

We deviate about plus 1m from the natural terrain slope, the neighbor about -2m...

So a 3m height difference? The original post says otherwise. Please also remember the drainage issue, especially with L-shaped stones.

Ever done something like this before? 1.2 meters with a foundation is easily 6m3 of concrete, you have to mix that first, then move the stones – that builds muscle.

My tip: dig 60-80cm deep. 40cm of gravel, then at least 20cm of concrete, have that delivered and set the first row directly into the wet concrete. Then after a few days, lay the next rows and fill with the mixer.

If you’re fast, you’ll have it finished in a week.

Then it doesn't have to be you who places the stones, but the neighbor.

So 3m height difference? The initial post says otherwise.

I am filling up about 1 meter and I have to retain that. The other person has excavated about 2 meters and still has to retain that. He hasn't done that yet. But why should I do that on his property?

Ever done something like that? 1.2 meters with a foundation is easily 6m3 of concrete, you first have to mix that, then move the stones, that builds muscles.

Yes, I’ve done it for 3m before, it took 1.5 hours of work, at least for the filling.

I am filling about 1 meter and I have to support that. The other person has dug out about 2 meters and still has to support this. He hasn’t done that yet. But why should I do that on his property?

The problem is that the forces of your retaining structure will affect his retaining structure. Unless you space them far apart, but surely no one wants that. If you both build close to the boundary, it’s also questionable whether the lower one can still build if the upper one has already built. For a total height of 3 meters, this needs to be properly planned and calculated.