My experience is as follows:
Back then, we looked at houses from three different developers (the price for each included the land, and the house would have had about 110 to 120 m2 of living space each).
The first house was offered turnkey for 289,000 euros. Upon closer inspection, there was a gap of about 80,000 euros between the offer and our understanding of turnkey, excluding the garden. With a garden, this house would have cost about 390,000 euros.
The second offer for house + land was about 350,000 euros - however, without exterior facilities, incidental construction costs, connection fees, and without itemization. The final price was not clearly predictable in advance.
The third one was a multi-page offer including every single socket, baseboard, and everything, including the construction portable toilet for the workers. The offer price here was 500,000 euros plus/minus 10%.
The most reliable offer for us was the third, as it was the most comprehensive and also listed many items we had not considered. Based on these three offers, I am very skeptical about offers from developers. We had also done some optimistic calculations back then, but ultimately we stayed away when we realized that we have to be honest with ourselves if we don’t want to live financially on the edge for the next years.
At first, we also wanted a fixed guest room, ideally with a small private bathroom. Ultimately, however, this is pure luxury that costs a small fortune. We have since outsourced this room. Our residential complex has two holiday apartments. Here, the night costs 55 euros and four people can be accommodated each. Relatives cannot come so often in a year that the guest room with its own bathroom would be cheaper...
Continue to save equity, improve your income situation, and look for a property in your price range.