The single-family house that you then sell as turnkey with outdoor facilities is priced at 485,000€ including the plot (living area 155sqm, plot size 698sqm, 1 garage, 1 parking space). The semi-detached house is priced at 460,000€ (145sqm living area, plot size for 1 semi-detached house 390sqm, 1 garage, 1 parking space) both houses with basements.
Sorry, but something is wrong here. You are supposed to pay 160,000€ for 720m² of land. That results in 222€/m². But the architect is selling 390m² and 698m² of land for basically the same price. The construction costs of the two houses are not much different. Overall, the final price from the architect does not fit with your land price at all. The calculation should look something like this:
Price single-family house:
698*222 = 154,956
155m² * 2000 = 310,000
Basement = 50,000
Garage + parking space = 15,000
Additional construction costs = 30,000
Total 559,956€
And this is calculated quite tightly. Either your land is massively overpriced or you are comparing with non-comparable locations.