Dumping soil on a garage roof – simply possible?

We have a boundary garage (actually there are 2 but only one of them is on the boundary) with a flat roof. Since we don’t have that much usable level space anyway, we now wanted to use the roof sensibly. Preferably one half as a terrace and the other as a green roof with lawn and wildflowers as well as a garden shed. The terrace is apparently okay in terms of permits (had an on-site meeting with the building authority) as long as it remains 3 m away from the boundary, i.e. the terrace may only have one roof. Although that doesn’t quite make sense to me, since the garage is completely built into the slope and the ground surface at the back rises about 2 meters above the garage roof, the front edge of the garage protrudes 2.5 meters above the street surface, so it’s quite a steep slope.
Accordingly, the terrace would at least be underground at the boundary up to half its area and therefore no problem?
But let’s say it may indeed only be built on the one roof, then the other should have a green roof.

Now, after much talking, to the actual point. It is a precast reinforced concrete garage for which we have the static calculations here. I wanted to know whether a terrace or an extensive green roof can be supported structurally, but apparently according to the brochure one may also put 45 cm of soil on the roof OR even place another garage on top (and with a car then, yes...). I can hardly imagine that I’m just allowed to dump 45 cm of soil on top like that now? And how do you secure it at the front (the sides are less of a problem due to the slope), just a row of planter stones and then horridoh?

Can someone tell me from experience that it really is that simple, because I can manage to dump soil on the roof myself. Is any permit required for this? So a change of use? The federal state is Hesse.
That would be just too good to be true if it really is that easy.

And the garden shed will be considered for itself (would be permitted as boundary construction) and at the back on the boundary garage it wouldn’t even be above ground. Or does that always mean a change of use of the garage? But a walkable green roof is also a change of use, isn’t it?

Just a question. Do you really want to have a terrace there?

It's an ideal place after all, what do you think speaks against it?

No, definitely do not just put earth on it. Firstly, that is the wrong construction to plant anything meaningful on it. Secondly, a regular prefabricated garage is not sufficiently dimensioned to bear the necessary load. Take a look at the documents for the garage to see what load it can handle. Otherwise, you need a structural engineer to calculate it beforehand.

You will not be allowed to build a garden house on the garage within the 3m zone.

Yes, as I said, the brochure (= "Official Static Type Check") for the garage states that even a second garage could be placed on top, or 45 cm of soil. So that means the roof load would be several thousand kilos accordingly. That surprised me too; I actually thought a simple wooden terrace might possibly be a problem. For example, it says "Traffic load on roof fill": 5kN/m2, which corresponds to about 500 kg per m2, right? And that is only the load ON the roof fill, which itself weighs a few kilos if it is actually 45 cm high... or am I completely misunderstanding this?

500kg/m2 is already a range in which extensive greenery becomes possible. Better ask a structural engineer. The whole thing probably won't be able to proceed without approval anyway.