Interesting topic. Like Joedreck, I also think that 100k will not be enough for an extension.
The question is which walls are going to be removed and what the new room layout should be, because there will be some pipes (electricity, water, sewage) in all those walls. If you are unlucky, all of that will have to be changed.
Also not to be underestimated are the hundreds of little things that hardly stand out during the first walkthrough. One example: When I look at the open-plan kitchen (EG-08), the tile pattern on the floor is ugly, there are surface-mounted pipes (heating?), and a glass door. These are points that I would personally change just in this room alone. But maybe you can live with it fine, that saves money.
Can you draw a floor plan showing how you would like to remodel the house? Where which rooms should be, especially the kitchen and bathrooms?