Estimate of construction costs for a single-family house in the Rhine-Main area

1.5 floors with 180 sqm living space + basement... If you calculate the VAT on that price, it could work. We are starting our construction in a few weeks, also in the RM area. We have no fancy stuff. With a garage and 156 sqm living space, 2 full floors. And according to the architect's estimate, we are a little over €340,000. Excl. fireplace, outdoor facilities, and incidental construction costs

I used the standard land values as a guideline, but I want to be prepared in case the seller demands a significantly higher price. Any other suggestions?

It's difficult to say anything definite about the land here. Currently, in desirable locations, it is a seller's market. You can of course try to negotiate. If the developer currently has 15 potential buyers, they will take the path of least resistance and will not make any price concessions.