Garage insulation is not mountable

To dig, I would need the neighbor's area. He has already planted there and laid paving stones.

You can do it properly now, or differently, but then it’s just ... :P The insufficient construction planning (if it was even possible at all to apply the sealing before the neighbor’s paving and garden work) only makes it more expensive, but not impossible. What could still be clarified: What was agreed upon, what does the architect actually owe you? What does the detailed planning of the garage actually provide here (sealing, insulation) and why?

The property ends with the concrete wall of the garage. To dig, I would need the neighbor's area.

Anyone planning a garage on the boundary must check whether the neighboring property matches in terms of height.

He has already planted and laid paving stones there.

Then the planner must be prepared to restore it. If he botched coordinating that, he must cover the additional costs.

That's why the question whether something can be done from the inside.

No, as already said in #8, that is not possible. Omitting the waterproofing against penetrating surface water precisely where it applies in particular (because, as you can clearly see, it does not infiltrate neatly vertically but first runs down the garage wall) would be foolish. To "trap" this trickle from inside would only lead to a damp wall. Structurally, I see no alternative to open trenching from the neighbor's side. The insulation must be carried all the way down; there is no way around it. Google hammer right. You must exempt the neighbor from all costs and have your planner, if he missed it, take care of the neighbor’s exterior work. The insulation can always be applied only on the fully built wall — one must be able to expect an architect to have the spatial imagination for the consequences this has for an earth-bermed garage.

From the outside, you actually can't get there anymore. Only from the inside could something still be done.

Ladder and hammer strike rights apply here. Apparently, the architect is building his first house.