Hi , sorry for the really long silence....
it's progressing - slowly but surely!
a) With the living spaces in the basement we are at ~176 m²
b) Last week
we received the building permit :)
There is still a lot to do - I need to write an update for the forum at my leisure.
Currently, I am concerned about the financing question: We are aiming roughly for 1200€ as a monthly installment and have always planned the option of special repayments....
a) ....preferably take a better interest rate for 15 years or
b) fix the current low interest rate level for 20 years (or even longer?)?
a) - the question here is what interest rate one can get for the remaining 226 k€ as a follow-up loan

b) - here I pay almost 12 k€ more interest over the comparable period, which corresponds roughly to an average year of principal repayment
For 25 years the interest rate rises somewhat more strongly to ~1.25%. For a full repayment loan (about 32 years) you also gain no advantages.