General recommendations/opinions/concerns

Regarding the floor space, we planned the garage relatively large, possibly with an attic. Furthermore, my parents have a big house where things can be stored that are not needed all the time.

And if a guest ever has to sleep on your sofa, you ring your parents and ask for a blanket.
If you plan a house for a family, you should also plan the house so that you are not dependent on your parents.
The house is currently too much of a pipe dream for your budget – despite [EL]. Materials also have to be purchased. Anyway: life involves more than a sofa and TV, also an ironing board, cleaning supplies, and filing folders.
The design is expandable. Better than many others. Nevertheless, it needs to be scaled down to fit the budget. It’s not a trick to fit 3 rooms plus bathroom and kitchen into 180 sqm.
By the way, my house plans looked exactly the same (a long time ago).

With an ensuite bathroom to the bedroom on the upper floor, only visually and thus not really acoustically separated by a sliding door, one hears all the farting, peeing, and plopping noises of the dear partner in action at night. Do you want that?

Want to build such a big house and take things to the parents for storage or safekeeping? That's ridiculous...

It can't be that bad if the small storage spaces are the biggest problem ;) If my parents live 1.5 km away and are visited at least once or twice a week anyway, I don't find it ridiculous to store Christmas decorations in the summer and the wheels in the winter there, we simply tried to create the largest possible living space with the same area/same costs that is always used, instead of areas for guest blankets that are used once a year. For general things like drinks, old clothes, cleaning supplies, toys, etc., I think the extension, pantry, attic, etc. are sufficient. We also discussed the "hotel-style" bath-bedroom in detail and are aware of noise, humidity, etc. there as well.

Personal price limit for the house, including equipment: with building plot 450K

Then let's calculate the other way around:
450k - building plot = 400k
-50k ancillary construction costs = 350k
-20k buffer = 330k
-30k garage (without this outbuilding!) = 300k
/1900 (Due to own work no 2000) = 157m²
you have about 9x12m house => 8.5x11.5m house then you get about 160m².

Put kitchen - dining table - living room on one horizontal line and make an office/guest room where the kitchen is (=storage).

best regards

He doesn't need storage space :p :D