So, when I read all of this, the €2000 is definitely out of the question. A demolition/change (at least that's how I see it) would be necessary but legally very, very difficult to enforce (proportionality). Let's clarify what an acceptable renovation solution costs? €20,000 - €30,000?? Can one speak of proportionality here?
Over the weekend, we noticed that the internal width of the double garage is too short (20cm).
Where exactly are the 20cm missing? In NI the rule is: either exactly on the boundary or at least 1m distance. If the 20cm are missing at the boundary, a solution must still be found for that.
I couldn't resist and just measured (on my Golf 7 / Sportsvan). The door stands exactly 1m ! beyond the outer edge when it's fully opened! Not 70cm or so. That would have been fun anyway. No matter if 20cm are missing.
Pragmatic solutions have already been suggested (sell the second car, get an eBike like Babboe ...).
it is "only" about moving a wall by approx. 20 cm.
Specifically: The floor slab probably needs to be enlarged. Not quite easy, but feasible. The ring beam could be reinforced with a "coating2 and a new ring beam could also be connected with the old one. Build up a new wall and done. The entire garage does not have to be torn down and rebuilt. The bricks have not yet been built up. So, definitely feasible in terms of cost/benefit. Compared to that, a garage that is too small, which you will be annoyed with for the next 30, 40 years. Because it is rather unlikely that cars will get smaller.
If you stick to the narrow version and absolutely want to park both cars inside it: park one forwards, one backwards, then there is enough comfortable space to get in the middle, passengers just get in after pulling out of the garage. It is not maximally comfortable but you can still get used to it quite well.