Construction costs for terraces, etc. in cost estimation according to DIN 27

It remains hard to swallow that, cost-wise, it makes no difference whether I convert the terrace 1:1 into normal living space or not

No offense, but you are kind of refusing to understand it. I know, stupid, yet you are asking for advice and have been repeatedly asked to post a sketch (which can also be done with paper and pencil) and to provide some numbers. This is total groping in the dark here. And that's exactly the kind of well-founded answers you get. Furthermore, it seems you somehow have difficulties grasping the building structure. Because your above statement is simply not correct. You compare 40 sqm of covered terrace with 20 sqm of living space. That's already not 1:1. Let alone the additional requirements for statics. As I said, really no offense, but if you just don't want to admit that your wish won’t come true, then you can end the thread at this point. If you want to deal with it objectively... provide more info. Then you will get, on the one hand, more comprehensible answers as to why it is so expensive, whether the proportionality fits, and even better, maybe tips on how it can be cheaper and still at least part of your idea can be implemented. Give it a try.

Most people start building their house with a dream castle, but in reality, there is just a small cube.

I can only repeat myself, your cost estimate is not worth the paper it's written on. Bet on it! Our house builder now calculates within his one-year price guarantee for the planning/construction phase with an increase rate of 10% for the individual trades, because at the moment, what is happening cost-wise on site is simply insane.

Take the planning and slap it down on a house builder’s desk and ask him for an offer, just to get back to reality in the first place. I suspect any thought of a terrace will then take care of itself. Sounds dumb, but unfortunately, this is the current situation on the market.