Is the planned construction of a garage on the property boundary allowed?

Now there could be several. Setback from the road, permission in a residential area on such a little field would hardly be granted. Through road or dead-end street.

Yes, there is a development plan.

And, do you want to reveal it to the forum? But we can also continue forming if ... then sentences and expand the questionnaire.

However, no garage is planned on the property according to this.

What does that mean? Is there no area explicitly designated for parking spaces? Is there a building boundary / building line? Is the property outside the buildable area? Are garages and parking spaces excluded outside buildable areas in the development plan?

I am not aware of a parking space statute, on the other hand.

Yes, that’s funny. Apparently, Gelsenkirchen is waiting for the statewide regulation from the ministry, but has enacted a parking space buy-off statute. So you don’t know how many parking spaces you have to create, but you know how much it costs if you don’t create them.
Well, it certainly doesn’t help you.

It is supposed to be a standard prefabricated garage

So 3m*6m=18m² < 30m² => exempt from approval

each 3m wide and high

Average wall height not over 3m => no setback area
With that, your defense rights as a neighbor are exhausted. Even if there are still building law reasons against the construction of the garage, you can only hope that the approval authority takes up your objections. Unlikely with a successful and influential developer.

Is it allowed to build a garage without a house?
Is it allowed to build a garage that, according to the sketch, covers more than 70% of the area of the small plot?

I’m afraid you are allowed to. Why is the construction company doing that? I’m afraid it’s to create a threatening scenario for the OP and make that small piece of land extremely expensive. Such a prefabricated garage with a metal door costs max 5 thousand. Some tenant paying 50 a month will be found. Decent return, but actually peanuts for such a lion....

If you were offered to rent the garage to be built, it might be possible for you to lease the [Grundstück] instead. Then the [Grundstückseigentümer] can monetize the [Grundstück] without you having to worry about a garage.

But it is also a question of whether one can simply get used to it. In any case, I consider the fear of burglars to be exaggerated, and you can provide light with lamps. You might also be able to erect a gate if you are afraid of strays on the driveway.