backerboard installation

In order to keep height down, do I need to remove subfloor before installing backerboard and tile so they the floor will be somewhat level with the adjoining floors.

You don't mean the subfloor, you mean the underlayment. All depends how the home was built. Probably yes. Tell us what you've got there.

Jaz

I think it is 1/2 or 3/4 inch particle board.

Particle board or osb? Saw dust or larger wafers glued together? When you look up from the basement, do you see plywood or planks for your subfloor, which is what is fastened to the joists and on which your walls are placed.

If it's particle board, definitely yes. If your subfloor is planks, you will need to go back with 1/2" plywood before you can install cement board.