I don’t buy an Audi and then get some Audi with some equipment. Besides, you have the chance, for example, to remove or add one or the other extra without additional cost
So, speaking in terms of cars: no, you can’t remove the air conditioning and add the sunroof with a nearly new car. A developer house is basically like a nearly new car that is freshly built for you, which is why you get it with 0 km.
The fact that a developer house is usually built only after the purchase does
not primarily happen so that you can still make changes, but simply for entrepreneurial reasons of the developer. Quite a few developers are also classic custom-build contractors and do the developer business to handle at least part of their revenue without dealing with the quarreling of laypeople, just because the latter are “king customer.” It’s about turning stones into money – just like in a brothel, it’s not about true love.