Is my house building plan realistic?

You should create your own standard of living and not measure yourself by the (house) size of your parents. You are in a good position regarding savings volume, but only average in terms of salary. I have read: 2400 € and calculated plus parental allowance. The equity makes an average single-family house affordable, nothing more. Four rooms of 15 sqm plus two/three bathrooms make 75 sqm. Living and cooking generously 60 sqm, with utility room, freezer, and ancillary areas you are well estimated at 160. Anything above that results in too high a demand that would be tedious to pay off.

From my personal point of view, a downsizing compared to, for example, one's own parental home is not necessarily a deterioration. Maybe a "few" m² less simply means less space to clutter, less cleaning effort/work, and ultimately a bit more free time and quality of life. Provided, of course, that it is not too cramped! From a cost perspective, it should be clear anyway...