The IBA Hamburg is building another new development right next to the new development area he mentioned. The information would already be interesting, especially regarding surveying and such.
Surveying costs are 400 bucks for him... Let's say 200 more now. No matter how precisely you want to calculate the incidental construction costs, it will never work. Inquire about the major cost drivers - utility connections, possibly more earthworks due to the slope or poor soil report, exterior facilities. For everything else, take some prices from current construction blogs or find the corresponding values online. (Land acquisition, notary, realtor) Then keep some cash aside as a buffer and you're done... Overall, you can be happy if you’re not off by five figures.