Ok. Thanks first of all. However, I didn't really understand option 1. Sounds very complicated. Selling the house now at a good price would of course also be a solution. However, in that case a prepayment penalty would definitely be due and we would already need a plot of land that is paid for, etc.
so I would first try to find a plot of land here, without a plot of land I wouldn't worry.
once a plot of land is found, it can be financed with a variable loan (through the house bank where the other loan is running), so financing based on collateral wouldn't be an issue here.
then take care of offers for house construction, incidental costs, outdoor facilities, etc. Once you have the numbers, calculate what makes more sense (i.e. whether to take over the old loan for the new house, finance the rest, and sell the house). or you would also have the option to sell the house immediately, with handover/payment in 12-15 months (construction phase). Sure, that would restrict the pool of buyers, but the "smart" people know that the house will probably cost 5% more in one year. Because with that, you have exact numbers to calculate with.
if you want to sell the house only after completion of the new construction, you would of course 1. finance a bit more as a buffer, and 2. should use the proceeds from the sale for special repayment.
So it would also be possible without prepayment penalty if you change the house...