Anonymous wrote:I'm not sure I understand the worry about water damange? Isnt the plywood stacked between wooden studs, but will be covered by cementboard and tile with fiberglass on top?
An important thing to be aware of in this project:
Cementboard is not waterproof.
Tile is not waterproof.
Grout is not waterproof.
Caulk is not waterproof.
Trim is not waterproof.
A shower enclosure needs to be waterproofed. This means using either a flexible membrane under the cement board, or a fluid applied membrane over the cement board. Anything else is going to leak. Leaks will cause your house to rot. This is a much bigger deal than adjusting the size of the opening.
Similarly, what are they doing on the outside? Windows need flashing around the top and sides or they will leak, also causing your house to rot. Doing this properly means removing about a foot of siding on all sides, flashing the opening and reinstalling the siding. Very commonly it isn't done properly.