Single-family house on the south slope with basement and ground floor (150-160 sqm)

after our first draft is not realistic from a budget perspective even after the first received offers,

Oh, wouldn’t aiming for fewer instead of more square meters be more logical?
Add a plot illustration with height information.

[*]If I were you, I would think carefully about widening that, making it bigger is always the "easiest" solution, but it also brings a chain of costs and the entire symmetry axes / statics can change. You are changing the whole house, not just the basement. I think you have enough space. We also considered it when our rooms were too small, but quickly abandoned the idea, as the plot is not endlessly large either.
[*]If you leave out the shower in the guest bathroom on the ground floor and add the space to the storage room, you save quite a bit and gain space. I have received feedback from many builders that they might shower in the guest bathroom 2-3 times within a year and that thing is basically wasted space that has to be cleaned additionally and was expensive.
[*]In the basement, I would shift things so that the rooms on the left become a bit bigger, while the ones on the right naturally become a bit smaller.
[*]What I don’t quite understand are 2x living/dining areas on the ground floor + upper floor. Do you really think you will carry the food upstairs to eat there in the long run? I rather see potential for more guest / leisure / storage rooms there. I would question the terrace upstairs the same way. You do have one on the ground floor as well. (Costs!) --> but prepare an expansion option.

[*]If I were you, I would think carefully about widening it; making it bigger is always the "easiest" solution, but it also brings a whole host of costs and the entire symmetry axis / statics can change. You are changing the entire house, not just the basement. I think you have enough space. We also considered it when our rooms were too small, but quickly moved away from that because the plot is not endlessly large.
[*]If you leave out the shower in the guest bathroom on the ground floor and add the space to the storage room, you save a lot and gain space. I have received feedback from many builders that they may shower 2-3 times in the guest bathroom within 1 year and that thing is basically wasted space that has to be cleaned additionally and was expensive.
[*]In the basement, I would shift so that the rooms on the left become a bit bigger, but those on the right naturally become smaller.
[*]What I don’t quite understand are 2x living/dining areas on the ground floor + upper floor. Do you really think that over time you will carry the food upstairs to the upper floor for dining? I see more potential there for additional guest / leisure / storage rooms. I would similarly question the terrace upstairs. You have one on the ground floor as well. (Costs!) --> However, prepare for an expansion option.