From when does "building" count?

Well, but obviously it took such a statement before you were ready to describe more comprehensively what you actually intend to do. Your first response to Scout was, I think, the first tentative description of your project...

, before you were ready to describe more comprehensively what you actually intend

All in all, I would say he wants to try to cleverly live somewhere as cheaply and legally as possible in a cabin.

Move to a campsite or put a construction trailer on a recreational plot.

What speaks against the mobile home and caravan solution?

I just want to briefly point out that various forest kindergartens with construction trailers as emergency weather protection still have problems with building regulations.

But please tell the whole story:

A trailer with a construction trailer body is a vehicle.
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1. A deregistered and only conditionally drivable vehicle can relatively easily be assumed to correspond to real estate because it can no longer be used/allowed as a vehicle in road traffic according to its intended use.
2. A forest is not meant for parking deregistered old vehicles under ecological and nature conservation aspects.

No one can do anything against temporarily parking a legal and drivable vehicle on private land, whether it is a building plot or forest.

For the OP that means: organize a used caravan, take it to the TÜV every 2 years, and then he can "park" it wherever he wants. Waste water, especially from chemical toilets, must be collected and properly disposed of.

Putting an old construction trailer down, preferably removing the wheels so that the entrance is not so uncomfortably high, and permanently connecting utilities will not work, as it then corresponds again to real estate.

State building code of your federal state?

For example, for Bavaria:

1Structural installations are connected to the ground, installations made from building products. 2Fixed installations for commercial advertising (advertising installations) including machines are structural installations. 3Installations that are intended to be used predominantly in a fixed location according to their purpose are considered structural installations, as well as
1.
Embankments, insofar as they are not a direct result of excavations,
2.
Storage areas, parking spaces, and exhibition areas,
3.
Campsites and weekend sites,
4.
Leisure and amusement parks,
5.
Parking spaces for motor vehicles.

So as long as you do not build a storage or campsite with it, no...

Is it sufficient if you build a shed without approval? That would be under 20 sqm here in Bavaria/municipality for me. Basically a concrete slab and a 4x5m wooden shed on top.

I am asking explicitly because there is a building obligation of 3 years on the property, whether you can meet this by doing so and then build the house only after 5 years?

Is it enough to build a shed that does not require a permit? [...] I ask explicitly because there is a building obligation of 3 years on the property, whether you can comply with it this way and then only build the house in 5 years?

*LOL* You are just hijacking a thread with a fitting title but a completely different intention.