Lowering the curb with barriers in NRW - How to invoice?

If the garage has a building permit, it naturally needs an access road.

That is not an argument. If you are allowed to legally construct a parking space without a permit, it naturally also needs an access road. There is a ruling from the Administrative Court of Cologne dated 18.01.2016 - 18 K 1119/15. Here, the plaintiff who wishes a lowering does not succeed with his lawsuit, but the reasons for rejection apparently do not apply to your case. Especially paragraph 23 is interesting. Opposing your request is the fact that you already have two parking spaces (with the garage even three). If a conversation with the head of department or office yields nothing, it is worth consulting a specialist lawyer. Edit: If it really only fails because of the missing garage, then simply submit a building application for a double garage with appropriate storage space facing the street. After you have made the lowering and the forecourt, unfortunately, the construction progress comes to a halt. In any case, this is cheaper than a court process with an uncertain outcome. However, you should have the approval in writing that the lowering is permitted with the approved garage.

And here we go. Attached is the rejection. Opinions? :/

Hmm, since it is legally perfectly justified, you don’t have a proper point of attack. With your plot size, of course, it’s annoying, but it’s understandable that such a regulation is generally made. Otherwise, for example, one could prevent someone from parking there by lowering the entire width of the plot...

The way you say it, it would make sense. However, parking is not allowed on the street anyway, as otherwise the road width would be too narrow for the fire department, garbage collection, etc. to get through. Currently, cars are parked illegally half on the sidewalk, half on the street. Two fewer cars could be parked there...

What does "two additional access roads" actually mean? I thought only one additional access road.

A driveway, but 5m long, so that two cars can park side by side.