I had already written... my first post on this was only meant as a "joke"
Since the construction project is far, far away from my completely normal construction project, I of course cannot and do not want to say anything about it. However, I also think the tip with an architect for the corresponding single-family house is very good.
Otherwise, just one more thought: Unless there are a few more million stashed away, I would really think carefully about whether I should not finance part of it externally after all. As a big fan of equity capital, I see this as an exception. Having a lot of liquidity definitely allows you to sleep better even with a lot of credit, and with a lot of liquidity you can react very nicely to certain market situations. Keyword: concentration risk. We now have well over 150k left, which we are not putting into the house or early repayment. Who knows what that might be good for someday. In the next crash (and it is due), I would pump a not small part of that into the stock market. Also, from a tax perspective, a loan of >1 million or more should be interesting. You have to somehow make the property commercially "usable."
Of course, these are all thoughts that are not relevant for the "normal" home builder. I count myself among them. After intensive discussions with my tax office, I decided on full private financing, even though an office is coming into the house. For us, the ratio between "noticeable taxes" and "taxable possible risk" in a later sale and removal from the company would not have been good. It always depends on the individual case.
Regards, Steffen