can character controller be done by me? how to create a step offset feature

it seems to me that charactercontroller could be done by me too and if one would do it by itself then it could suit it's needs much more if done correctly so am going the wrong way if i want to create sg. similiar to a charactercontroller or maybe one using physics rather? how would i add make it so that it can only go up steps of a certain height and below? thanks for everyone in advance for trying to help dissolve my confusion regarding these...things :)

There is a rigidbody physics version of the FPS walker on the wiki - here:

Physics FPS Walker

This may be just what you're looking for. Or, if you really want to write your own, reading through the scripts on that page would probably give you some good inspiration.