Extension to an existing house, costs, experiences

Hello,
We are planning to add an extension to my parents' house.
We are already living upstairs in the house with 3 rooms and would like to extend so that we have 4-5 rooms. It would be about 70 sqm extension, 1.5 stories.
What should one calculate for that? 1000 € per sqm?
Has anyone ever built an extension?

Best regards

So significantly cheaper than the calculation for a new building you will not get. Connecting to an existing building always means extra effort - from simple to considerable. On the other hand, you can hardly save anything. Such an extension can cost 100-200k€. Certainly not 70.