Shelf should be a solid pc of stone (curb or countertop match preferably) not whatever that pc is there now.

What do you plan to put on this little area? It seems too small to be fun tho al

Functional.

It's unfortunate that there is such a thick center shelf piece vs. using just a shelf. I would remove that thick red shelf. I think that you should be able to remove it + add appropriate waterproofing. You could then line the top/sides of the niche with the same material as used for the bottom of the niche + use it for your shelf.

Below is a similar style niche as yours with the thick shelf = wastes space inside the niche:

The above niche appears to use tile for the top/sides/bottom of each of the niche spaces with a Schluter type trim in white to finish the edges of the tile.

I would use the same type of material to cover the sides/top/bottom + also use it for the actual shelf. The thinner shelf = more usable space inside the niche + you are still at the stage where you can install a thin shelf (it sits between the top and bottom side pieces used).

How tall is your niche (overall height of your niche) + the height of each space above and below the thicker prefab shelf piece? It looks like it would be tights to be able to fit large shower/conditioner bottles with pumps in either of the spaces. If that's true, you would be able to divide the two spaces differently = more functional for you.

Example:

Below is a niche that uses some natural stone material (tiles) to outline the entire niche + for the shelf. Also, the shelf is positioned in order to provide a small area at the bottom for soap/razors/small loofah/a cream style of women's shaving cream/a short container of extra hydrating hair mask/small face wash/etc. + the upper portion of the niche has more height to allow for large shampoo/conditioner pump bottles. Tip - don't place a bar of soap directly on the bottom of the niche use a soap holder that is small and allows the bar of soap to dry between uses = no mushy bars of soap or ones that are stuck to the shelf.

The following are all types of soap holders (these are all from Etsy - but Amazon has similar items also).

These are made out of cedar wood = better for getting wet and drying over and over.

Below are "Quick Dry Diatomite Soap Dishes" - they allow the soap to dry quickly = not mushy.

They have the below type holder with or without the holder below it.

I have a similar niche in one of my guest bathrooms - I decanted shampoo/conditioner/body wash into nice looking large pump bottles (there are many types/styles/colors + have various labels in order to identify the product inside each of the matching bottles with pumps - no glass - but amber ones that are made out of nice looking non glass material = no glass on shower floor if one of the bottles is knocked off the shelf and breaks.

The above top/side/bottom solid pieces sit just a tiny bit proud of the shower walls. You don't have to have yours completed that way - some people prefer for the surround pieces to be flush with the wall tile - others prefer having more of a "framed" look for the niche.

Here is one that used zellige tiles for the shower walls/back of niche/sides of niche/top of the niche (mitered edges were used) + used solid natural stone for the bottom of the niche and the shelf (I assume that it matches the stone that was used on top of the shower curb). The division of the niche was left with two matching spaces = same height. It just depends on what works better for you personally.

Here is another niche that appears to have used the same solid material for the top of the bench seat (and I imagine the same material that was used on top of the curb. The pieces outlining the niche has also been installed just a bit proud of the field tile (wall tile) which creates a framed look. The niche was also evenly divided into two spaces (again, whatever works for you personally).

Here is another niche that was constructed the way your contractor initially planned on doing your niche + with a thick shelf that is tiled over) - however, the contractor moved the shelf piece down a bit in order to have more room for shampoo/conditioner bottles in the upper portion of the niche with a shorter sized portion at the bottom of the niche for storing small items.

What material was used for the bottom of the niche? That same solid material could be used all the way around the niche + for the shelf. You will have more usable space inside the niche by removing the thick prefab shelf. Hopefully, you can have pieces fabricated to fit the sides/top/shelf of the niche to match the bottom piece. Using the same material as will be on top of the shower curb + the countertop for the vanity is a popular way of completing these types of niches. However, they don't have to match your countertop - as long as the solid pieces used to trim out the niche + top the curb work well with your field tile + the countertop material you've selected for you vanity.

Thanks for the suggestions so far. To answer some questions:

@millworkman The shelf is made of solid stone threshold, the closest color I could find that matched the tile. I'm open to other ideas. It isn't installed yet, just sitting there. I can't find those thinner pieces that I see pros use. I've been told they are at Home Depot, but I've check many times in multiple locations and can't find them. Although now that I look at the picture, maybe those are the same thickness.

@WestCoast Hopeful The niches are 16"x14", plenty big for large shampoo and soap bottles.

@dani_m08 The red part is redguard, it hasn't been tiled yet. It is not a shelf, it is 2 separate niches, exactly like your first picture. Since they used small tile, they were able to continue it onto the middle.

They are 16x14". I do plan to use schluter on the sides and tile on the sides and top. But since I used a wavy tile, I can't use it for the bottom. I thought about just using a glossy white tile, but liked the way the stone gave it a little more space to put things on like one of your pics has as well. Countertop is also white. I don't have a contractor, I do my own work. Which is why I occasionally need advice! haha!

I'm still trying to figure out what to do with the in between section. I might just be overthinking because this seems logical:

My first thought is whatever material you used on the bottom of the niche should be used for both sides of the divider as well as all the inside side walls with your accent tile just on the back walls of the niche.

IMO the same material as the shower walls was the best but even better no niche .Niches are a PITA to get right and for sure shampoo bottles are not decor and do not need to be high lighted .

Would suggest that you use the same tile on the wall to finish out the niche. You tile installer should easily be able to do that. Would not us schlutter trim if you can avoid it.

"The shelf is made of solid stone threshold, the closest color I could find that matched the tile."

Then why are we looking at Redguard? The solid surface shelf should be exposed and visible, that spot does not get tiled the shelf becomes the divider and would pitched correctly and actually a bit proud of the niche, exactly the same as the niche bottom.

@millworkman Oh, you mean the center section? It is 2 separate niches. I like the look of that like some of the above pictures are showing. I thought you meant that bottom piece I have sitting there.

@Norwood Architects I am the installer. I can't use it on the bottom of the niche because it is wavy. How would you finish it without doing miters? I didn't trust that I could do miters well enough so already planned on schluter.

" It is 2 separate niches. "

Then the piece you have separating the 2 niches is not thick enough for tile it will look off.

@millworkman It's just over 2" tall just like the above pic I posted.

Looks nice!