I belive it’s a hardie backer board.

Waterproofing goes over Hardie Backer as the board is not waterproof. There really should be a waterproof membrane (Kerdi or similar) wrapped around the corners and into and behind the window frame on all 4 sides. A one piece sill will be nice but not add to any waterproofing the same as tile and grout are not waterproof.

If that is a shower the waterproofing has to go up the full length of the walls and not stop at the window ledge height

That ^^^^^ too.

Not completely sure from the photo, but I hope the window is vinyl/composite/etc and not wood. I would definitely get a new window if this one is wood.

chispa - what do you mean when you say the window is vinyl? do you mean the area around it?

What is the window material built from? Wood can not be used inside a shower. It will rot. It has to be a vinyl or fiberglass window. Or glass block. Lots of technical issues with keeping water out of your house. Windows are not designed to be waterproof from the interior.

It appears to be a vinyl window with a drywall extension jamb.

The frame around the glass window is certainly not wood. But that frame is most likely stuck to drywall supported by wood. Hope that answers your question.

The contractor says he will take the inclined one-piece quartz window sill all the way to the glass window. Has anyone done that before?

But what that do? I mean a one piece sill will shed the water off the sill better (if properly pitched) but it will not help or do anything for waterproofing anything.

Oh, there will be waterproofing precautions underneath the sill. H's trying to minimize the number of gaps and caulking areas as much as possible.

" Oh, there will be waterproofing precautions underneath the sill. "

Ok, just looking to be sure as you have been asked about waterproofing more than once and have given zero inclination or idea of what said waterproofing will be.

what about waterproofing on the walls?

@millworkman - The entire area, from ceiling to floor, all three walls including around the window - will be waterproofed. The contractor said he will be using Hardi backer. However based on one of the comments here, I will ensure he uses a waterproof membrane like kerdi as well.

AB Normal, thanks for expressing your concerns. I sought feedback, not judgment. If you have further insights or suggestions, I'd be grateful to hear them.

A shower curtain immediately in front of the window.

I used a paint-on waterproofing on my window sill (actually all 4 sides of the window opening) and on the cement wall board surrounding the shower as well. Most have heard of Red Guard for doing this, I used a similar TEC product:

https://www.menards.com/main/flooring-rugs/tile-stone/tile-installation-maintenance/tile-waterproofing-surface-prep-systems/tec-reg-hydraflex-trade-waterproofing-crack-isolation-membrane-1-gal/7002745011/p-1512978148199-c-6590.htm?exp=false

We then installed a sloped sill (similar to Corian) and caulked it to the window frame with clear silicone. IMO, it sounds like your contractor has the right idea. The remaining 3 sides of the window opening had cement board installed, water-prooofed with the TEC product, and then ceramic tile installed using thinset and grout.

This blogger attached a piece of plexiglass with a few dots of caulk. It seems like a good solution to keep the bulk of the water out, while still allowing light in and air to circulate.

https://victoriaelizabethbarnes.com/window-in-shower/

@AB Normal, it completely depends on the contractor.