Register a building burden retroactively on your own property?

What happens if neighbor No. 42 gets the legalization and the building encumbrance registration approved? Am I then forced to build the house on the other side by neighbor No. 38? There are also windows here, but certainly no fire protection windows like at No. 42. Do both neighbors then have to get the legalization/registration approved so that one can ultimately rebuild?

Do both neighbors have to get the legalization/registration approved so that you are ultimately allowed to rebuild?

That doesn't necessarily have to be connected. If you register the building encumbrance with both neighbors, your property in the front can no longer be built on (13m - 2x3m building encumbrance - 2x3m own setback area = 1m permissible house width).
However, that does not mean that development at the back will be approved. In the worst case, you end up with a property that can only be used as garden land and is therefore almost worthless. So don't be too quick to offer this registration to both neighbors!