Basement bathroom rough in old house

I am trying to help my friend look into roughing in a bathroom in an old house that she just bought. Not sure if it's worth it. I am fairly knowledgeable but the fact that the house is old scares me a bit. I don't have any experience dealing with the old metal fittings and I know nothing about how the main stack attaches to the sewer line to the city sewage.

The basics.

House is about 50 years old. Maybe a more. There is 1 bathroom on the main floor which is actually just above where she is thinking to put the bathroom downstairs.

The drain from that bathroom is metal, I am guessing cast iron but I suppose it could be lead. It runs in the floor joists to the end of the house where the laundry area is and down the wall into the floor. I am assuming that is the main sewer connection. Here is the confusing part for me. The bottom 3 feet or so of the foundation wall is a bit thicker than the rest of it and the stack is actually buried in the concrete. I am only able to see a little bit of the side of the 3" pipe. The 'Y' cleanout however is sticking right out and fully accessable. The laundry tub also runs right into the contrete and dissapears from view. I really wish I had taken a picture. below the 'Y' fitting though, the pipe is completely burried in the concrete and not visible. I would assume it continues down into the floor. The problem is I don't know what these connections look like.

Here are my questions.

-What do I need to be concered with if I were to rough in a bathroom in an old house? From what I have seen on TV some of these main sewer lines used to be clay pipes, they were often broken and a big mess to deal with once the floor gets ripped up.
-How complicated/difficult is it to tap into the main drain once the floor is ripped up.
-Is there a limit on the length of run from where we want the bathroom to where the main sewer connection is? It's about 18-20 feet right now.

I am basically thinking about cutting a nice big trough on the floor and digging it out, then paying a plumber to come in and do all the pipe properly and I will fill it back in myself. Any thoughts as to what someone may charge to do this type of work if the grunt stuff was already done? Not sure if I am allowed to ask, but is there anyone in the GTA that may be willing to come do it?

It would likely be best to install new DWV for the new bathroom, and then only have 1 connection to the old system under the slab.

I figured I would be installing a new system under the current slab. My concern is attaching it to the old system, how to do it and why the current stack is burried in a concrete wall.

Normally one would cut out a section of the main horizontal building drain and install a wye in it's place. This could be done in the basement or out in the yard.

I would not speculate as to why anyone would have done the concrete as you describe. Some things only make sense to the person that did them.

quote; I am basically thinking about cutting a nice big trough on the floor and digging it out, then paying a plumber to come in and do all the pipe properly and I will fill it back in myself. Any thoughts as to what someone may charge to do this type of work if the grunt stuff was already done? Not sure if I am allowed to ask, but is there anyone in the GTA that may be willing to come do it?

You pay the plumber FIRST, the he tells you where to cut the floor and dig it up, assuming he can figure it out before you start. That is NOT an "old house" and it would be very unlikely to have clay tile pipes under the floor. Having a bathroom above where you want to install the new one is not necessarily a benefit, and could even make it more difficult.

50 years is old? Time to move to Florida and put on the white shoes and straw hat. Christs sakes, I have shoes older than that

Thanks for the insight. I'm 27, and around here new houses are going up like lego. Mattemy had a house factory up the street from me that built fully detached 2000 sq ft + houses on an assembly line then brought them out on a truck complete with all fixtures in place and dropped them on a foundation. So to me 50-60 years is old. I don't see much metal supply lines anymore nevermind metal drains. That's why the thought of clay pipes scares me.

I did call a plumber and he did say that he would want to come in and see the area before I start chipping so thank you for that. He even gave me a rough starting price of $750 including materials and with him chipping the floor (me filling it back in) for about a 20 foot run to the bathroom. How does that sound?

hj, would it be worth having him replace the current accessable metal drains in the bathroom above if he ends up having to replace part of the drain stack? I mean, he's there anyway, and an extra couple hundred bucks isn't much to make the upstairs bathroom reno easier for me when it comes time down the road...