Gravel bed over property boundary - is approval necessary?

3rd option: the bottom edge of your base plate is a little lower? How much? As a compromise, your neighbor should join a strip of subgrade (gravel packing) 50cm wide along the boundary for your base plate. OK, gravel on your property is the bottom edge of the base plate. You finish the rest later and then pour concrete. But honestly, such things are not discussed in a forum, as already mentioned no irregularities should occur (shafts...).

5cm, ours 25cm, his 20cm Thanks, that’s not a bad idea. Then we could build an extension afterwards. I agree with you, this will lead to several rounds of discussions here. We unexpectedly received the request to agree at short notice without knowing what it actually means, and it was perhaps quite good to have a first contact point to get a feeling for it. ;)

If you knew the reasons for "not getting a move on,"

The reasons don't care about reality.

That we must not damage their base plate is obvious.

No, you only half understood that: the whole ensemble only works if planned and executed in a concerted manner.

Unfortunately, we do not yet have a building permit... otherwise, we would have no problem with it being included.

Then your refusal to consent is unavoidable, as the former chancellor would have said. But explain it peacefully to the neighbors. Where does the problem come from (on the one hand their haste, and on the other your still pending approval)?

That means if Plan A "shared base plate" doesn't work, Plan B "strip foundation 50cm deeper than our base plate" would also be possible?

Them stepping down is also part of the concerted planning.

5cm, our 25cm, his 20 cm

And what would be the heights of the top edges for those?

I would handle it very pragmatically. Have the consent declaration given to you as a Word document. Then you add (or have it added by a construction lawyer) that all additional costs for you (shoring, special measures, or whatever) are borne by the neighbor. Maybe write a flat rate maximum of T€ 50. And the matter will be settled...

And what would be the heights of the top edges for that?

Can you read it from the pictures before I misinterpret something?

I would deal with it quite pragmatically. [...] And the issue will be resolved...

As Plan B in case of stubbornness, yes. But for proportionality, your understanding should be attainable without triangulating over an even greater threat. What expected due date of your approval are we even talking about?

Can you read it from the pictures before I misinterpret something?

The white (20 cm) base plate would have its top edge at 89.25 and the red (25 cm) at 89.00, so your base plate would start 30 cm deeper.