Semi-detached house in NRW, what costs?

The costs for the house depend on your requirements, your own work, and local regulations. Quoting a rough price doesn’t help much. With a budget of 400T€ you can safely start making more detailed inquiries. What comes out of it at this point in time is just speculation.

400 for house + land, plus about 70k buffer. At 2000/sqm that is still tight? What is still missing in the calculation?

Well, first of all, the question is whether you really want to do without a basement given the size of the house. If I hypothetically subtract it from the mentioned project, I still come to 350,000 EUR just for the semi-detached house – plus floors and walls. (Since this is a developer project, there are no additional incidental building costs.) If you then add your buffer, you are at 420 K just for the house. I'm not saying it can't be cheaper, but the developer there is rather large and, in my perception, already builds cost-effectively, so I don't believe it will go significantly lower. In addition, the price per square meter is higher the smaller the floor area is.

We don't want a basement because of the high groundwater. We are less confident about awarding the trades separately, as we simply lack the expertise for that. The requirements are rather low to normal, I would say, but yes, that's hard to judge.

So according to my calculation, the construction including ancillary costs should be around 300k, so that the remaining buffer still lets me sleep well. That would be a total investment of about 415k including the property. Buffer about 55k, which will probably shrink faster than you'd like.

What about the local requirements? Such a soakaway costs something, photovoltaics can also be mandatory (of course it's worth it, but initially it burdens liquidity), there can also be requirements for the exterior and roof appearance that hit the budget. Problematic are plots where the owners can be involved in road renovations. "We develop for you and you pay for the road later..." would be bad. Ask about that carefully!

Sorry, somehow I don't understand what you want to do with your buffer now. Is it added on top or should the project mathematically keep this buffer to the 415 K? The latter seems hopeless to me with the property, the former can work if the buffer can be used without much hassle. If you also forgo a basement due to soil conditions, the foundation could still turn out more expensive than planned. In case of doubt, a generous buffer can quickly - sorry - evaporate.