New toilet and sink in the basement

Hello, I am working on adding a bathroom in the basement with just a toilet and a sink. The place where I want to put the toilet is 3 to 4 feet away from a PVC stack that feeds a toilet upstairs. I want to attach the toilet to the horizontal pipe under the basement floor that the stack feeds into. I know I need a combination wye with a 1/8 bend and possibly a vent for the toilet. I was wondering if anyone has any diagrams that could explain how to run everything. I have a good amount of experience in plumbing but this type of job is a first for me.

One other thing I was thinking of is if I tapped into the vertical stack for the toilet and went with a raised floor in the bathroom instead of tapping into the horizontal pipe under the floor. I'm assuming the first way is the right way to do it but I didn't want to rule it out until I talked to someone. Thanks for your help.

The best way is to crack concrete and tap into the drain below the floor. Raising the toilet always looks like an afterthought handyman's solution. While not my favorite, something like a Saniflow pump would allow you to do this without breaking the floor. Gravity is your friend, and the preferred way. You do need to vent both the toilet and sink, though. You'll need to find a way to get that vent pipe up through the wall above the highest point that is a drain (normally, this is spec'ed to be 42" above the flood rim of the highest device that is draining into the pipe). You could go all the way to the attic, or even through the roof, if it works out best.

Definitely agree with Jim. Breaking concrete in a basement is not the most pleasant job in the world, but it can be done. If you want to avoid the dust and hassle of the job, hire a professional concrete cutting company. They will do a very fast, very clean job, far better than you could accomplish by just breaking the space, and they will clean up the mess. Don't take the easy way out by building a platform. You will regret it forever. Having broken out concrete in my basement for the same purpose, I can tell you that to do over, I'd hire the pros and find something else to save a few dollars on. The job ended up OK, but it took several evenings of hammering with my hammer-drill and it created so much dust, it took weeks to clean up the mess.

I really wasn't serious about the platform idea, it was just a thought. So would the toilet go right above the horizontal pipe with the combo? Could the vent go on the horizontal drain between the stack and the combo?

You need to study a bit on how a vent is installed. There are some diagrams here that should help.

As to where to install the toilet, some of this depends on how deep the line is and your preference. You need a bend on the drain to then connect (typically that connection would be a Y) to the drain line. Depending on how deep the current line is, it could be problematic. You need room for the toilet bend, the Y and then 1/4" per foot to the drain line. So, without checking, it's hard to say how easy it will be.

The vent needs to go up from the device being vented. You can combine vents above the flood rim of the vanity sink so you only need to run one vent from there up to where you can then join into an existing one, or run it through the roof on its own.