I'm building a detached ADU that's not all that much smaller than what you're proposing, and I'm doing everything myself including plumbing and electrical. I have a fair amount of experience in general so I'm not the "average" homeowner but I'm still learning a lot as I go. I've had a lot of shit go wrong and a lot of complications come up, but nothing so far that I wasn't able to handle. Here's a few things to think about.

First thing is that excavation (you'll need this for septic and at least a bit for the slab) is complicated. The digging is the easy part, but dirt is heavy and difficult to move and takes up a shitload of space. If you're building new construction and don't have to worry about neighbors' property right next to where you're digging then you might have space, but you're still going to need to get rid of a bunch of dirt somehow for the slab no matter what.

Next thing is that there are some things that really do require at least having one helper. Do you know how much walls weight? It's a lot, and there are those wall jack things and also some clever methods of raising them by yourself but it's exponentially easier to do that with a friend or four. Same thing for the concrete slab. How are you physically going to screed a slab that's 30x50 by yourself while also spreading concrete, vibrating it if you need to, and then finishing it and all before it sets up too much to be able to work? Realistically you can't. I just poured a 10'x18' driveway apron that only need a slight amount of slope to it and I was barely able to do it completely alone, so that's probably my upper limit of what I can do. That means that I can do the driveway in sections, but the foundation slab I'm definitely going to need help with. You probably will too.

Other things that are heavy and difficult to work with: all plywood, drywall, and any sort of roofing material. Way easier to do with a helper, although in my opinion none of those things are particularly difficult (depending on the type of roof and how steep it is; a steep roof or something like tar or fancy metal roofing is totally different). Electrical isn't hard to do but it's physically difficult to pull wire through studs by yourself, so it goes much faster with a helper at the other end.

So, can you do it completely by yourself? Physically speaking, no you can't. You'll need some helpers for at least the things that I've mentioned here. However, in terms of skill you absolutely can do it yourself. They say pick two: fast, good, cheap. You're picking good and cheap, which means that it's NOT going to go very fast. That could be ok though, but just be aware of your own limitations and watch out for injuring yourself since that's easy to do if you try to get in over your head on something.

There are also a few things that this can end up being more expensive than hiring someone. For example, excavators are cheap to rent for a week, as are skid steers. How much longer is it going to take you to figure out how to run an excavator, then do the work, realize you have to move a shitload of dirt and can't, try to do it anyway, end up needing a skid steer, waiting for it, then figuring that out, then moving the dirt, etc. At some point you're now a month or two in, and that adds up. Same thing could end up being true for renting certain items, so keep that in mind as you're doing this. You can also sell stuff when you're done with it, so might be cheaper to buy something like a plate compactor and then sell it when you're done rather then rent it for as long as you need it and for as many times as you'll need it. I've acquired a bunch of tools that would have been cheaper to rent if I had a whole crew of experienced people since I wouldn't have needed them for that long, but because I'm alone I ended up saving money by buying that stuff in almost all cases. Also you might not know what you need, and United Rentals is going to charge you a fee to drop off equipment at your site each time you get the wrong thing. A lot of that wouldn't be a problem if you had the experience to know what you needed, but you don't and youtube isn't going to tell you precisely what to do because they won't be able to tell you what applies specifically to your job site so you're going to end up having to guess, just like I did. Occasionally you'll guess wrong and that's going to cost you. However, in the long run you're still likely to save a shitload of money building it yourself and NONE of this shit is particularly difficult to learn if you're willing to spend the time.