Prefabricated garage foundation in the workspace

Hello everyone,
the basement was constructed for my house building project. This week the earthworks contractor was supposed to fill the workroom.
Problem: a double prefabricated garage will later be placed directly next to the house. That means we have a foundation directly in the workroom.
Question:
how should I construct the foundation?
The depth in the workroom is at least 2.5m. A foundation with a height of 2.5m will be really expensive.
How have you solved this problem?
THANKS for your help.

Fill the work area layer by layer with gravel and compact it up to a height of approximately 1m below the top of the foundation. Then you can form the foundation, pour concrete, and subsequently backfill the rest.

I spoke with my building inspector today. He suggested 2 solutions:
- fill one side completely with gravel and compact it in layers. Then foundation on top.
- Alternative: foundation, formwork stone wall, foundation. Here a mason offered me the price of €2200 (foundation, wall 6.5m long, total about 2.8m) and then 2 more foundations about 40cm high for €900.
What do you think of the solution and price of solution 2?