I am renovating the downstairs bathroom. Concrete floor. I am adding a 2x4 subfloor with plywood on top then tile. I need to raise the flange about 2 1/2 inches. Can I just use extension kits for this? I really don't want to break up the concrete floor to modify the flange.

With enough extenders,(see images ) and a wax seal,(see image) with a funnel you might just make it. Good luck, Ton