What else is added to this object?

If I hire an architect, I have to pay him for the drawing, then I give it to the building authority and they say, for example, no, that won't work, then I paid him in vain, right? I thought I'd just have the construction company come right away, and after we've talked about the price and so on, they should do the sketch!?

You shouldn't just look at the number at the bottom right of the offer. You want to realize a complex construction project (an extension to an existing house). That requires a lot of expertise in the planning phase. It's much more difficult than just slapping a standard 0815 house somewhere. Invest the money in good planning – you will benefit from it. And if it doesn't work out, at least you'll know. Why should a developer create a plan for you – without charging anything? They also have costs until a sketch is done – if the construction doesn't happen, they will charge you for those costs.