House construction on existing land - Financing model

I am not a fan of the building savings contract. You pay in a significant amount now, but you get (almost) no interest on this money. Instead, you pay the debit interest on the still outstanding amount - which could have been lower if you had put the money for the savings phase of the building savings contract directly into the repayment. So the good interest rate is somewhat relativized. In addition, it is probably relatively inflexible... although it will probably be possible to make a special repayment during the loan phase of the building savings contract.

After all, in the end, it comes down to pretty much the same thing regarding the paid installments (under 10k€ difference, actually in favor of the building savings contract, as already wrote). I would personally probably choose option 2, especially if it is conceivable to make a special repayment within the first 10 years. If you could come up with about 20k€ (does not have to be all at once), you have already offset the difference to option 1 in the form of saved interest.

With the home savings contract, however, definitely avoid the Riester variant. If the granny flat has a separate loan, it is easier to deal with the tax office.