Assessment "Fixed Price" Detached Single-Family House (Solid)

200sqm and 3 wet areas are average? Is that sarcasm? Nothing else is average either.

It says "larger" than average ;-) If you can pay 600k + land + double garage + outdoor facilities + incidental construction costs + incidental acquisition costs, so over 1 million, you naturally expect more than average. We also find it shocking how little we get for our 800k, but unfortunately, that's just how it is.

That's true about the terraces. The neighbor has 3 of them and only uses the garden in the front because that's the sunny side. The other sun terraces are "dead."

Just a waste of money. Otherwise, definitely think carefully about [Dine] like the sauna. If the budget allows, I would really include it.

That does not sound like a normal single-family house but like a villa.

No, to me the description reads more like an amateur plan, with the floor area inflated by awkward arrangements (and possibly additionally symmetry, a straight staircase, and the rest of the usual suspects). At the latest with certain buzzwords, it must ring a bell with the experienced reader that something like this is probably being planned:

[*]The incidental building costs were stated as €69,000

Fixed price €658,000 (excluding land, incidental building costs, white tank, garage, outdoor facilities, kitchen)

What does that mean?
If I take several variations of a calculation, I always come to normal construction prices (under €3,300/sqm)!
!!Turnkey prices/offers DO NOT include incidental building costs, outdoor facilities or kitchen, unless it is a BT house, where everything (except furniture) is included.
Therefore: everything is within the usual range!

For the guest room, I always think about the costs and the effort involved in maintenance.... how many 5-star overnight stays do you think you could offer your guests before the guest room has paid for itself? We built completely without a guest room and get strange looks for it :-P

When I think about the house, I also always think "better to have than to need," but 200m² of living space is really something. With your ideas (sauna, multiple bathrooms, roof terraces, garages, garden, ...) I already see close to a seven-figure amount, since the architect's offer is just the bare basics for now.

If you have budget concerns, you should reconsider your plans: What do you really need? Can the sauna be put in the basement, for example? Otherwise, you'll be driving your dream house straight into the wall from the very start. Life is better in a 150 m² house than in a 200 m² house where money is regularly argued about and every cent has to be turned twice.