Unequal width in the planned semi-detached house

Thanks again for the answers. The development plan requires pitched roofs and profile uniformity. So that already fits. The other party has now suggested installing a retaining wall during the earthworks for the base slab to save the costs of later support. However, we would have to bear the costs for the retaining wall. We would prefer to persuade the two to be smart and simply build with a basement as well, but the contract for the earthworks is probably already signed.

We would most like to persuade the two to be smart and simply build with a basement as well,

Now let's keep things in perspective: that wouldn't be "smart," but "your smart." "Their smart" is probably something else. "Objectively smart" would only be building with a basement if an appropriate slope required it.

The other party has now suggested to erect a retaining wall during the earthworks for the foundation slab and thus save the costs of later reinforcement. However, we would have to bear the costs for the retaining wall.

You probably have a naive idea of statics together. The short meaning without long talk is: you’d best also build without a basement (if the property votes "without basement" according to the 11ant basement rule).

The development plan requires gable roofs and profile uniformity. So that's fine as it is.

We would most like to persuade the two to be smart and simply build with a basement as well,

I would rather try to increase the roof pitch, if possible. Financially, that doesn't make as much of a difference as building a basement in addition. If necessary, I would offer to share the additional costs.

The thing with persuading was, of course, a joke. Nevertheless, it would somehow be nice if not only one side had to make compromises in a joint construction project. The basement is non-negotiable for us. Would a partial basement possibly be feasible that leaves some distance to the property boundary? I don't know how much distance would be necessary here, 1-2 meters? Would that be less complicated? Then the basement would not connect to the neighbors' property but would be completely under our house.

The thing about persuading was obviously a joke. Still, it would somehow be nice if in a joint construction project not only one side had to make compromises.
The basement is non-negotiable for us.
Would a partial basement possibly be an option that leaves some distance from the property boundary? I don’t know how much distance would be necessary here, 1-2 meters? Would that be less complicated? Then the basement would not connect to the neighbor's property but would be completely under our house.

I would discuss such things with the architect or general contractor.

The thing with persuasion was of course a joke. Still, it would somehow be nice if, in a joint construction project, not only one side had to make compromises.

Your neighbors have not promised you shared suffering before any altar!

The basement is non-negotiable for us.

Absolutely. "If the lottery fairy doesn’t draw my numbers, I’ll die immediately" – such a fatalistic attitude is unhealthy and will never have the strength to force supposed luck. By the way, you haven’t commented on the slope location yet.

Would a partial basement perhaps be possible that leaves some distance to the property boundary? I don’t know how much distance would be necessary, 1-2 meters? Would that then be less complicated? Then the basement would not adjoin the neighbors’ property, but would be completely underneath our house.

I deliberately only mentioned the brief point without a long explanation because a partial basement is possible but uneconomical. The distance must also take into account the slope of the excavation pit, so 2m would be roughly the lower limit (and your staircase would thereby be fixed to the side away from the neighbor, although it would actually be considerably cheaper on the “shared” wall side). This Pyrrhic victory is not really necessary. Rent space in a self-storage center if you still need time for sorting flea market / recycling yard. A basement for junk capacity is by no means the only option.

I would rather try to raise the roof pitch, if possible. That doesn’t make as big a financial difference as building a basement additionally. If necessary, I would offer to share the additional costs.

If a longer house length is allowed, that would be easier, then you could abandon profile equality in the additional length. But I do not really see this as necessary here.