Someone butchered that. Rip it all out and don’t look back. It’s more likely to be a standard size than a non standard size. Especially considering the weird box out of the plumbing wall. But until demo happens, you don’t know why everything was done so weirdly. Expect mold remediation to be a must. That should have been an alcove tub with an integral tiling flange, not a drop in tub. There will be moisture damage, no question.

Is there a tub elsewhere in the house? If so have you considered just making it a really nice shower?

@User Thank you for your advice. We don't mind ripping the whole tiles off and the demo will take place two weeks from now. There is definitely a reason why the previous owner kept the weird box. We talked to Kohler and they don't see a 60'' x 30'' alcove is working with part of tub tucked into the wall. Would love to hear your thought.

@lyfiaThanks. Well, that's the only place the house has a tub unless we redesign the house to create a new washroom.

Thank you @User. This is a pretty old house, I believe the bathroom was done about 15 years ago. There are no washrooms below or above. Not sure why there's a weird box.

How old is the house? I suspect a cast iron vent pipe hiding in that space on the left. Or perhaps heating/cooling ducts. It would be a lot of build out currently, but I see it as being entirely possible that a 60x30 tub was used depending on how wide the tub ledge is. Maybe they made it look pretty by tiling over existing tile to make it more sellable. If you take the faceplate off the shower valve you might be able to see if there are two layers of tile.

@mainenell Thanks, it's a 95-year old house. Believe the owner did the renovation roughly 15 years ago. I guess we'll find out what's inside the box once the demo is done (we'll rip off all existing tiles, our contractors told us these are natural stone).

With demo imminent, do go ahead and do a bit on the front end. Find out what’s inside the box. That will let you know what hidden change order $$$ you’re dealing with.

So lath and plaster probably. And vent stack or something else in there. It’ll be interesting for sure. Just as a side story, we took out a shower in one of our apartments. The previous owner had put in a 32” shower with a built out wall so that a one piece shower could be used. My plan was a 36” preformed shower pan with tile. So we took out the shower, the built out wall, a layer of Sheetrock and strapping on each side, a layer of lath and plaster on each side and ended up with 42” available for the shower pan! I reordered the shower pan. I no longer have prospective tenants saying they won’t fit in the shower! (They wouldn’t have! It was awful. Shower head was actually in the height of the shower so you also had to be short!) So, hopefully, you will have as positive a result from your rip out as we did!

There's always a first on Houzz.. this might be one issue I've never seen come across. I vote on ripping it out.

Ok i've seen this set up before in a home I redid . They added a bathroom so modern plumbing was put up and walled in (the wall bumpout) then they did a raised drop in tub to cover the tub drain line pipe that ran back to the new plumbing wall. This is a hack. The new contractor should be able to reroute the drain line into the floor and this would allow alcove tub which would fit the space The new plumber may be able to bury the lines in the wall. This looks like a cheap quick remodel. I've seen this set up in 2nd floors and on concrete basement where installing new plumbing is costly. Hopefully your remodel will go quick

Are you sure that’s a drop-in tub?

@Sammy no we do not know for sure. it might be alcove installation tub without skirt

Please post pics so we can see what this looks like ripped out, we are all curious!

Thanks for the update.

What is behind the bump out?