Designing Kitchens and Bathrooms - Installing shower enclosure

I am wanting to add a shower to an already existing tub enclosure. I would like to know the easiest way to add to the tile that is already on the wall, up about 18 inches above the tub, so that I can put a shower in. :rolleyes:

The "best" way would be to strip the existing tile and sheetrock off of the wall, install Wonderboard, or some other concrete backer board, then install new tile all the way to the ceiling. You will run into several problems if you try to just add to the existing tile. First, can you match it? Next, the top course is probably a bull nosed tile and it will have to be removed all the way around the tub. Also, at the valve end of the tub, you are going to have to open up the wall to install the riser for the shower head. Once you get that opened, you will know whether you can use the existing valve, or will have to install a new one. Some valves are 'tub only' and have no provisions for adding the riser. (sometimes, things are not as simple as they appear!)