Of course, you can also build for less. My house construction (approx. 130sqm) in the extended Lake Constance region is calculated at around 545k, I added a little reserve to the financing. But everything is included, and some items from the extra cost list are checked: KfW40+, geothermal heating, continuously tiled flat showers, garden shed, and carport with tiled roof. But some things are not included: basement, carport instead of garage. Own contributions are only outside; I do nothing inside the house (not a professional and the construction site is an unfamily-friendly 45km away). However, I take care of the entire outdoor facilities: garden shed, carport, paving, splash protection, etc. Most of this can also be done well after moving in if the distances are short.
After deducting equity and grants, just under 400k remain, which must be repaid according to the rule of €400 per €100k of debt. At the follow-up financing in 10 years, a small part will also be paid off through a building savings contract (it still has 3% interest!), and then about 230k will remain. It will then depend on how the interest rates are. Possibly with an increase in installments, a full repayment in 10 years will work if interest rates remain low; otherwise, one in 15 years is aimed for. Total duration therefore 20-25 years. Although I am rather optimistic here, with a €2200 installment, the loan would be repaid in just under 10 years even at 3% interest, and if the interest rates rise that much, I also expect corresponding inflation and wage increases before then ;)
In my opinion, three things are important to avoid repaying forever: 1. not to build too expensively: no expensive land, not too big, no basement, no luxury frills 2. bring equity 3. repay properly