Can you increase height of toilet flange?

Extend the Wax Ring instead. The Fluidmaster No.35B Wax Extender Kit is specifically designed for problem areas where the floor flange is 1/4" to 1" below the surface as in tile work. The cost is under $6. Some Plumbing supply stores sell an extension that bolts to your existing flange. If you need an extension due to new tile work, this will do the trick. If the flange is only one quarter inch or less below floor level, you can just use two wax rings.