You need to fix this at the foundation before you do anything else.

13 degrees is a lot and is indicative of foundation stability issues. Before doing anything, you should verify that the foundation is currently stable, even after 100+ years it could still be sinking slowly, depending on the makeup of the ground below it. You level the floor, remodel the bathroom, and in 3 years you start to see cracks from the foundation sinking another 1/4 inch. There are ways to stabilize foundations that are not currently stable.

Additionally, I suspect mudjacking your house may make the most sense. This involves lifting the sunken part of the foundation up a bit and filling in the gap. This will naturally level out the floors and provide the most structural integrity overall. In some cases it's not as expensive as you may think and should be well worth it if the foundation has finally settled. Not only are your floors more level, but your house is more structurally intact overall. The downside is it's going to cause cracking in existing drywall, plaster, flooring etc. that was placed according to the sunken angle of your house, but like you said, it's a fixer upper, so presumably you're poised to do that work anyway.

At some point a house leaning due to a sunken foundation is dangerous, so it should not be taken lightly. The leaning tower of pisa is leaning at 4 degrees. It is 6 times older than your house and still sinking for the same reasons your house may be.

Can you put jacks under the subfloor? You'll have to examine all of that - there may also be foundation issues precluding that as an option

If you are gutting the bathroom remove the subfloor, sister the joists with level joists, then reinstall the subfloor.

If you are not gutting leveling compound or mortar and cement board is the way to go. A skilled tile man can nearly level a floor with varying thinset thickness.

Edit - I’ll echo the “try and identify the cause of sagging first” comments. I have a 100 year old house and things have naturally moved around a bit, but it’s mostly just dips between the joists and things being out of plumb. An entire room dipping so much is pretty sus and it could just happen again after you fix it if the problem is ongoing.

It's typical for baths in older homes to have rotten joists from decades of water. If this is that simple, jack up the joists, tear out and replace rotted sills and joists then sister or replace joists.

It isn't overly technical, just a bear in a crawlspace. Hopefully you have a basement.

In this instance you can’t level the floor because you have a sewer drain connecting to the toilet. So if you level the floor you will have to extend the sewer line and probably water line. If it’s a 100+ old house and it’s just a 1/2 bathroom then most likely an added portion. Houses that old did t have regular plumbing they had outhouses. Everyone has it right. You have to start with the foundation first. I know it sucks but without a solid and level foundation you’re just asking for more problems. Especially if you use leveling compound. That will add extra weight to that end of the foundation. You should really start with a Structural Engineer. They will do a complete house look over. Then make your decision off of that. If it’s just that room then it might be (depending on your level of expertise) easy for you to DIY the foundation. I’ve worked on old houses and sometimes the SE says oh there’s a small area and sometimes it’s oh shit you need to move out. Just being honest here. Good luck!

You can cut shims from 2x4s and then lay plywood on top of them to create a level floor. This will raise the height of your floor slightly. You could also use leveling compound, but for that amount of slope you will have a lot of buildup on the downhill side, and it says heavy as concrete.

You could use leveling compound with filler pieces of cement board to make the process go more quickly. Cement board might have air pockets in it, so it'll be a little lighter.

A lot of these answeres are good, but you really need to find out why it is sagging, and if the reason it is settling is finished. Like did something rot? Is it still rotting? Did the foundation settle? Is it stillttlig. As for jacking back into position, vs leveling the floor, jacking may fix other problems, or may cause other problems, and if you level rhe floor, then have to jack the house later, then the floor will be out again. Fun stuff! Also look outside then house, is there a broken downspout, or some other water issue out there?

Find out why it's sagging and fix that issue first. Before you go Jacking up shit consider the fact that many times this will cause door closing opening issues as stress points are changing.

I feel like foundation needs air quotes. A house that old could be a stone and mortar foundation and it's crumbling. I agree with those saying to address that first.

[deleted]

A 13° slope is practically unusable. That amounts to a drop of 2¾" per foot, or a full step every 2.5 or 3 feet. On a small bathroom (since we don't have the dimensions) that could mean a drop of 1.5 to 2 feet.

You'll be replacing the whole floor, and much of the subfloor and piles, so you may as well decommission the bathroom and rip the flooring out. That'll be much easier than going into an underfloor crawl space and finding the sinkhole to hell is full of the contents of a broken sewer pipe.

Are you positive it's 13°? ... 1.3° would already be bad enough, but 13° is beginning to sound like someone just laid flooring on top of a [shallow] staircase.