New building with general contractor garage built too small, what to do?

Sorry, quote cut off:

The law of economics forbids getting a lot of value for little money. If you accept the lowest offer, you have to add something for the risk you are taking. And if you do that, then you also have enough money to pay for something better.

The error here will have been that various things were probably checked in between, but not the hole (i.e., the width of the gate opening) - so much for ongoing inspection. The mistake was probably made by the floor slab caster, or the floor slab stake layer. And afterwards, the incorrect cornerstone distance was simply continued correctly in itself.

The negligence itself will therefore have been low, and intent entirely absent. Nevertheless, I see the fulfillment of the contract for work here as missed by more than a "tolerance," and 19 cm narrower is no small matter. At least the gate – completely, and not just the additional cost for its dimensional change – should teach the general contractor the lesson of a future alcohol-free construction site.

Think again and imagine if in both cars comfortably driver and passenger plus children want to get out and/or get in,

Even families enthusiastic about synchronized dancing do not all get into both cars simultaneously with every man, woman, and child.

That is really a stupid situation. Whether a demolition is proportionate I of course cannot judge. But I hope you find a solution acceptable for you. Very curious.....

First of all, the contractor made a financial offer - clearly too low in my opinion, because the garage no longer serves its purpose as intended.

I don't understand why it wasn't measured beforehand, after all, you have to live with it forever.

I understand that very well, unlike you. Whoever checks and inspects everything might discover such errors. Instead of correcting others afterward, I prefer to make good progress myself by delegating with trust - I prefer to create and experience a lot rather than getting bogged down. Then something like this can happen, and I bear some responsibility as the one who delegated. A garage that is too narrow is a solvable problem and not a big deal.

In this case, you still have bicycles, shelves, garbage bins, or whatever standing next to it.

That's often the case: in practice, whether wider or narrower, a car will always be parked outside anyway, because stuff and children's vehicles will take up one side.

You write in your post about the expert. Has he already accompanied all the steps on the construction site? Ours measured everything again. (Certainly not the task of the client).

Was the fine staking done accurately? Was the base slab for the house and garage worked on together or did it come later?

A garage that is too narrow is a solvable problem and not a big deal.

Well, it is quite a bit of work. Tear down the wall and roof, tie into the floor slab (pretty ugly) and then rebuild wall and roof. Tying in 20cm is also hard to do – so you’d probably rather do 50cm or so? How does that look on the overall plan? Is there space for it and nothing in the way like pipes, etc.?
But I would do it too. Such a narrow garage is a nightmare.