So - securing the loan is not a problem (but talk to your parents about it so they don't fall from the clouds). (e.g. register a land charge on the house) In your place, I would also try to pay off the remaining loan on the condominium. (new land on the condominium)
With these securities, you should get almost perfect conditions (almost 100% secured). Or could the usufruct rights here be value-reducing - question to the credit experts among us bankers?
But now comes the difficulty: If you calculate with an average 5% annuity, with 500,000 EUR you would have a monthly rate of about 2,100 euros. As long as you both earn, this should not be a problem. But if children come along, it looks bad. You alone will no longer be able to afford the rate.
A few suggestions for a solution: 1. As long as you are an only child - your father could possibly support you with part of the stock fund (100-150,000 EUR - there would still be 200-250,000 EUR left as security for your parents). WARNING here: If you have already had the house transferred to you in advance, talk to a tax advisor beforehand. Keyword: gift and inheritance tax / allowances. So that you don't have to pay heavy taxes unnecessarily.
2. Build something smaller. Is the basement absolutely necessary? 35-40,000 EUR
3. You may have estimated the incidental costs ( ) and the buffer a bit too high. We currently have a thread running where this buffer would be useful, but I think it is very high. You will probably learn more about this during the construction companies' presentations.
PS: Very good that you did not include your wife's part-time job and commissions. This gives you an additional capital buffer of about 800 euros per month.