Is the price of the base plate justified?

Hi!
In our area (rural NRW), this would be significantly cheaper if you organize a bit:

- 1 day digging + delivery, installation, and compaction of 50 cubic meters of RC gravel approx. 2500€
- 1 cubic meter of concrete including delivery approx. 150€ (in your case about 15-20 cubic meters = 2250€-3000€?)
- a few working hours from 2 concrete builders/masons = 500€

I would commission it separately at my place, so the excavation work would be contracted out to a contractor or similar, for the concrete work try to find one or two people for half a day, and order the concrete yourself directly from the plant. If you pitch in yourself, you only need one "expert".
With formwork and reinforcement (material + installation), it might be half a day more (+2500€). So overall in the range of 8500€.
In 2020, I poured a floor slab for my place, but without excavation work (I dug by hand). That cost around 2500€ back then (material and labor) for a 59 sqm, 16 cm thick, and properly reinforced floor slab. All prices include VAT. And a concrete pump was also included.

I also wonder whether this is necessary and sensible. A soil report was created for the adjacent house (80-90cm is soil group OH topsoil).

For the house foundation, these were the recommended measures from the report (removal of topsoil and strip foundations). Whether this is necessary for a garage, I cannot assess either. We want to build the garage ourselves and had planned to have the floor slab done by the shell builder. Therefore, there are no requirements from a garage manufacturer.

Topsoil must be removed in any case. Topsoil initially contains humic components that can decompose over the years and is also considered protected material in Germany. However, 90 cm is already a rather high figure, of course that is not normal (20-40 cm is normal). A plowed field has 50-60 cm. But whatever, I am a fan of slab foundations anyway.

Do you have structural engineering plans for the garage? If yes, then just have the structural engineer calculate a slab foundation (which is not very complex). Then, as described above, use F 1 gravel, 1m cushion height, and skip the strip foundations.

Are you still in the construction phase? How is/was the house founded? Of course, that must also be taken into account...

90 cm is quite a high figure, of course that is not (20-40 cm is normal). A ploughed field has 50-60 cm.

Either someone dumped something there once or peat fertilization was carried out in the region.

Topsoil must be removed in any case. Topsoil initially contains humus components that can decompose over the years and is also considered a protected resource in Germany. However, 90 cm is quite a high figure, of course that’s not normal (20-40 cm is usual). A plowed field has 50-60 cm. But never mind, I’m a fan of slab foundations anyway.

Do you have a static calculation for the garage? If yes, just have the structural engineer calculate a slab foundation (it's not that complicated). Then, as described above, use F 1 gravel, 1 m cushion layer, and skip the strip foundations.

Are you still in construction? How is/was the house founded? That obviously must be taken into account...

- There was supposedly a nursery/tree nursery on the property at one time. Maybe that’s the reason.
- I have to request the static calculation. The planner created it together with the planning for the house.
- The house is under construction. The soil report also recommended (90 cm replacement, strip foundations 90 cm as frost protection).