I have the feeling that you are very fixated on your dreams, which is certainly not unusual or inherently bad at your age of 15. The only problem is that, in the end, the financial background has to be right in order to fulfill all these dreams.
I think if money didn’t matter, some would build differently (bigger, "more luxurious"). But if, as you wrote, the original version with an expanded attic is "too expensive" (whether that is because more cannot or should not be spent), then the costs do not change significantly just because you move the space planned upstairs downwards. So alternatives and compromises have to be made (whether that is another house shape, a different roof, maybe only one children's room that you temporarily share, etc... or maybe not even a new building but an existing property, that everyone has to decide for themselves). Or more money must be generated (if somehow possible and reasonable). In the end, one can also be happy with a house that does not correspond 100% to the dream ideas.