First of all, my condolences for the really unrealistic separate construction of the halves of your semi-detached house. I strongly recommend that you revise this dangerous wrong decision as far as possible, for example by at least starting your foundation slab at the same time. Explanations can be found in my post "A semi-detached house has TWO halves" (on "Bauen jetzt") as well as here in the #goalkeeperthread (which, although primarily about his beautiful house, unfortunately inevitably also involves Mayor Dumb and Neighbor Dumber, so the story works excellently as the leading required reading for everyone who can still be saved from the ruin of individual semi-detached house construction).
Now first the good news: concerns that your recycled gravel could be infected by normal gravel and lose its organic certification are unfounded. The misfits may play together and even sing the same songs. The master properties of gravel are grain size and compressive strength – whether the gravel comes from free-range or caged origin is irrelevant. One can never have enough gravel, quacks Uncle Scrooge.
Our architect said they want to have a strip foundation installed so that the two houses can stand autarkically,
Secondly, semi-detached halves never really stand autarkic from each other – geologically, Cain and Abel never avoid having to watch over their respective brother. They are at most decouplable – and that should be done – acoustically. And first of all, the architect may please state what he understands about foundations; I fear a novice without experience from responsible construction practice. Changing him now, almost "between the years," will probably be difficult. What kind of person is he anyway: self-chosen or an inclusive planner from your general contractor?
Our architect has concerns because, for example, the cushion cannot simply be removed, otherwise the cushion under the foundation slab would simply "run out" and the house would have to be braced.
At least he has a minimal basic understanding of the viscosity of the involved masses—there is still a glimmer of hope.
that their gravel cushion may protrude onto our property. Allegedly everything will be taken back after the is completed.
That is the actual core of the (fundamental) problem: that reality makes no cut at your shared boundary, and all forces act radiantly. Even their foundation would basically need a margin on the shared side, and the gravel cushion definitely would too as long as the half stands alone. The fact that their gravel cushion overhang on the shared side becomes dispensable as soon as your half is added allows for the "removal" of this part, which for anyone competent in bulk goods is an inevitably amusing word. That it would gradually be resumed at your construction start was already correctly pointed out by the neighbors, and now we come to the Achilles' heel of the catch:
Remarkable is also that the houses probably won’t be at the same height, but ours will be set deeper due to the thicker foundation slab.
"Remarkable" is the wrong word; it should be "must be taken into account," and "probably" is dangerous: these height details should be coordinated strictly. From your description, I deduce that at least on part of the height of the neighbor’s gravel cushion, your side’s foundation slab would already be their neighbor itself.
Under no circumstances allow this to fall into the planning and execution responsibility of separate contractors!!!
Therefore, my (factual and timely) most urgent advice is that you also have the excavation contractor provide your foundation slab—and at the same time. Also read here using the search term "Fundamenterder."
You are building onto an existing building. Then you must neither undermine their house nor let anything run out there. In excavation of your pit, the bottom of the neighbor’s foundation must remain covered with at least 50cm of undisturbed ground (see DIN 4123).
If your foundation slab lies deeper than the neighbor’s, then you must automatically underpin,
That’s exactly it. Therefore my advice, the two most urgent parts being: have the foundation slabs made together and consider the different heights
planned in detail(!).
So communicate with your neighbors that you were warned in time against building the individual halves separately and that you would rather join with them for the foundation than go this approval route.
In a divided billing of planning and construction measures, I see no problem. Likewise, you can plan and build separately—possibly delayed—once a safe height for the stability of their half is reached (see above).
--- here I was already finished when you wrote again ---
I think the neighbor obviously doesn’t have to take this into account, but if he can’t wait 3 months, then he has to make appropriate arrangements to carry it out "damage-free," that’s my naive view.
The view is indeed more than naive because the person obliged to show consideration here is
you.
That’s exactly how we see it too, that with the desired procedure something would "run out"...
(see above)
Do I understand correctly that you also think a strip foundation is the only reasonable way? Of course, we don’t want to alienate the neighbors, so we have to proceed carefully :/
Forget the childish chatter of your planner about foundations; he only half-listened in his studies. You should proceed not only
carefully, but above all
wisely, and not alienate yourselves _and_ the neighbors ;-)
In this sense: Congratulations on coming before the child fell into the well!