House Construction 2.0 - First Floor Plan Draft

I took the measurements from the floor plan you posted: e.g. window element "Kitchen/Dining/Living room" 2.145 x 3m (HxW). We have H 2.10m and it unfortunately looks lousy.

Oh wow, I hadn't even noticed that. I was waiting for the views. o_O

So, we have the first 20 - 25% price increase with that. I suspect there will be more, but of course, I'm keeping my fingers crossed for you with the bingo :D

Thanks :D So far, I've never had much luck with gambling; I think I have too much luck in love for that. :P

The price naturally matches much more with the price one would expect. However, in defense of the construction company, I can only assume that they really miscalculated and didn’t do it intentionally.
The consultant is genuinely likable. I’ve also met some who come across as very forced pseudo-likable, which is not really my thing.
Furthermore, I don’t see what they should gain from it. If my client says, "I want to build about 170-180 m² and have a budget of Y," then it’s not really a smart strategy to design something that is 100 m² larger and over 100k more expensive than planned.

He had underestimated the garage living space.
...
Maybe it would also be an option to build the house as drawn, without the garage part. And then make a second structure with a carport and a storage room underneath, but not as part of the main building, and accordingly without all the insulation and other residential building stuff.

Surprise ... not. And he probably severely underestimated it.
Honestly, what exactly is so attractive about endlessly long corridors and tiny rooms? I'm really not an advocate of mini hallways, but the other extreme is just as stupid. And what about the depth of the living room? That interests me – looks like under 4m.
Apart from that, the potential savings from omitting the insulation under the garage are probably not that big, so that the structure fits the budget in the end. Using the rooms under the garage is not a bad idea. They're just too good to be used as storage for garden tools. Using them as bedrooms is also questionable, as the cars above will surely not park silently. Now you can consider what options are feasible and gradually approach the plan. I see the office moving there – it’s much cheaper to buy an additional coffee machine than to force the proximity to the kitchen. The retro games – really cool hobby, but do they need 16 sqm? How big is the collection? It’s all very inflated and constantly needs to be cleaned. That’s practically already a burden. Therefore I find ...

... unless we finally win at bingo. :D

not even worth striving for.

And furthermore, I don't see what that should bring them. If my client says "I want to build about 170-180 m² and have a budget of Y," then strategically it’s not really a great idea to design something that is 100 m² bigger and costs over 100k more than planned.

Well. At a general contractor, dozens of clients come who want 160 sqm and can barely afford it with great difficulty. If then people come whose demands and income seem to fit better and want 170 sqm – which is then relatively less – then he might want to tease out whether, after a little appetizer, a bit more might be possible. Many salespeople also think I’m not as poor as I am stingy ;-)

And furthermore, I do not see what that should bring them. If my client says "I want to build about 170-180 m² and have a budget of Y," then strategically it’s not really clever to design something that is 100 m² larger and costs over 100k more than planned.

Did you go into the planning with your maximum budget?

The 590k does not yet include additional costs due to the slope. Make a plan for the entire property and have it priced accordingly. Then you will see why the slope is more expensive.

I am still waiting for meaningful documents regarding the property.