Adding a connected garage afterward - leave the cladding open?

Unfortunately, such details also influence wishful thinking, we unfortunately cannot turn that off.

Professionals have to master implementations that do not require the customers to be spoon-fed every detail. Of course, they have to comply with laws, including physical ones, but you don't have to provide them with fully elaborated feasibility proofs.

But wouldn't that also mean that the builder would then have to do the strip foundation and the garage floor slab as well?

The floor slab would have to be there if he is doing the masonry. Who does it is secondary.

However, the connection of the masonry would then be more problematic. Especially to make the whole thing watertight.

It must and will be made watertight even without connecting the masonry. The house and the garage must be thermally separated.

If I have read correctly that the garage is supposed to have a wooden roof structure: it should be relatively easy to leave the garage open to the house side without its own wall (?)

That's what I meant too: no extra wall for the later garage. Either the beams rest on the rear masonry of the house (a bad option for several reasons) or the facing bricks in that area are replaced by KS or another type of brick.

In our case, the facing bricks in the garage area were replaced by 17.5 Poroton, and accordingly the insulation in that area was reduced by about 6cm. The background to all this is that the facing bricks are theoretically not statically load-bearing. At the same time, however, you don't want 3 layers of masonry, at least I don't.