What is the best way to build a closet over existing carpet?

Answer 1

You could build on top of the existing carpet and this wouldn't cause you any problem, until you replace the carpet. I'm a carpet installer and have removed a lot of rugs that have walls, islands, bookcases, and cabinets built on top of them and this is a little irritating to deal with. Also when tearing out the old carpet, someone who is not careful can start tugging at the rug without realizing it is under the wall. This will cause your wall to shift and weaken, and crack your paint.
So, building on your subfloor is really the best idea. If you pull back the carpet to do this it will have to be restretched. You could buy a cheap knee kicker and do this yourself. Another option (for a small closet with two walls) is to leave the carpet where is is and cut out rectangles for your floor plates, leave the holes a little too small and slit the corners so you can fit the board in. If you do this, tack some nails around the edges of the holes first so that the carpet does not lose its stretch and wrinkle.

Answer 2

While I am no contractor, I would say that building it on top of the carpeting and pad is certainly not the best method. You want the walls to be rigid and secure. It will be quite difficult to nail the framing to the floor with carpet and pad between it and the floor.

The best thing to do, if possible, would be to pull the carpet away from the wall and roll it out of the way while you build the closet. Once it is completed you can simply roll it out again and cut it to the new dimensions, cutting away the waste. That way you have a new closet that is perfectly carpeted and would be much simpler than piecing the carpet.

I hope this helps.