Build the garage yourself - Calculate the statics?

That is 90 m2 of floor area. In terms of volume, you are planning no less than a house of typical size with 1 full story plus a roof. Just without technology and (except for the gate) without larger openings. Would you also trust yourself to do the shell construction for a house? I mean, you are asking about lintels and roof structure... have you ever gone through that in your mind?

Of course I went through that. But a house is somewhat more complex. Insulation/airtightness is not an issue for the garage, as long as water doesn't flow inside in streams, certainly nothing serious. After all, people always drive their wet cars in there in the rain.

I also thought about a ring beam, but I'm not sure if it can span a 6m+ clear space. Steel double-T beams might be possible too.

It's now more about brainstorming ideas so I know what is possible and sensible. The easiest would certainly be to build the garage with masonry, ring beam on top, and then have concrete slabs delivered that are placed directly on top. But those also probably need to be secured?

Oh dear, no offense meant, but get someone who has done this before. There is a lot of dangerous half-knowledge at play here. You will need a building application anyway, so also someone authorized to submit building plans. They can also show you options and costs. For the size, if you do it yourself, really everything, expect about 20-25k in material costs. Filigree ceilings will be too expensive for you, better plan towards wood. Your "lintel" will be part of the ring beam, iron according to statics, nothing rocket science, but small mistakes can be expensive to correct later.

Thank you for your insightful answer. I don't want to start building tomorrow without sense and reason, but also be able to weigh things up. For example, I have an offer for around 18,000 euros. However, the dimensions don't really suit me (9x9x2.55). It's really a lot of money for "just" a garage. Yes, I know the garage is huge, but we also have a lot of stuff and the thing also has to accommodate the basement junk.

Gladly. I have already gone through all of that myself. Mine is just slightly smaller. If you have yours built like that, it should be around +/- 35k. Tip: Proper planning (heights, drainage, materials, supply lines, ...) beforehand and get everything approved. Have the excavation, connections, gravel installation, and the base slab done in one go along with the house. Then persuade a bricklayer/groundworker to build up the garage on Saturdays or after work with 1-2 helpers you provide. The same goes for the roof, just with a roofer, then have it plastered together with the house. Doing everything yourself is not worth it; look carefully where it makes sense and make sure you always have competent people at hand to guide you. This way it goes fast, you have many synergy effects, and it’s not much more expensive than doing everything yourself.

Johanna von Koczian sang in my youth "that little household, takes care of itself, says my husband" - I always think of that whenever I read the overestimations of hobby masons who consider their garage not so complex and therefore suitable as a journeyman's piece. The engineers are regularly worse at this than the clerks, who at least are more aware of their non-expertise. First of all, tiling the utility room yourself calibrates that again.

As a hobby mason, I would initially recommend either aerated concrete or shuttering blocks instead of KS, both of which are considerably more theory-friendly to work with.

Omitting the central post just to save three or four shunting moves when repairing the car is something I would think twice about. And in any case, leave the planning to a carpenter from the top edge of the ring beam.

That you are now asking about the garage probably means that getting permission to build on the property in the noise protection restricted zone we talked about some time ago has worked out?