Floor plan and elevations for a single-family house of approximately 160 m² on a hillside location

And you think that you pay just as much for that as for an above-ground floor?

What reason would there be for the SQM of living space to be cheaper to construct underground than above ground?

That may apply to a utility basement, which is practically unheated and unfinished and used as storage space, but not for living space quality for two offices with guest room use.

Hello, I also want to say something about the windows. Really make sure that enough light can come into the house. So don’t just eliminate the loopholes, but also pay attention to the height of the windows. I don’t know the targeted room height, but 2.1xxx m seems very insufficient to me. Then there is clearly too much wall above the windows up to the ceiling.

The prices seem way over the top to me. A good architect doesn’t misestimate that much. Sometimes one quote is 10% higher and another 10% cheaper.

And you think you pay the same for that as for an above-ground floor?

Architect-designed houses take longer, so the calculated prices can no longer be maintained. HOAI also costs a small fortune, and prefabricated garages do not exist in this size and in this angled shape. Basement: fully within the thermal envelope with underground sealing combined with insulation and exterior plaster. Planned entirely as living space, the calculated cost is similar to above-ground living space.

You may be right about the basement prices if all rooms are developed as living cellars, BUT building with a good architect who has known all the craftsmen on site for years was easily possible until 2021, so you don't pay more than with the general contractor (GU) and the estimates for the individual service phases were exactly the same as the actual offers. Since 2021, the market has of course become crazier, but by now all architects should know the high price level and be able to calculate accordingly. You also don't pay the margin of the GU when building with an architect!

BUT building with a good architect,

No matter if good or bad. Building a house with an architect is subject to different conditions than with a general contractor… as already said.