where is my main drain line at?

hi,
my house is pretty strange. it is 2200 sq.ft. with 1 small bathroom. it has a partial basement, and the rest of the house is a slab. it used to be a small cottage that was added on to over the years.
This winter i would like to add a bathroom to this house, but i don't know where the drain lines are. The 1 bathroom and the kitchen in this house is built on the slab, and there are no drain lines in the basement that i can see. The place i want to install the bathroom is on the opposite end of the house.

here is how weird my house is, the sumppump in the basement discharges through a hose that is routed through the foundation wall, up through the slab under my sink, and into the same drain as my kitchen sink. weird huh? i found this out the first week we moved into this house, we ran the garbage disposer, which plugged up the drain. then we washed a load of laundry, which empties into the sumppump, the sumppump turned on, the force of the pump busted the pipe under the sink and flooded my kitchen. it was a mess.

so i need to know how to locate the main drain in my house. there is a small 1.5in pipe that looks like it could be a drain from my kitchen sink, but if you read my previous post about finding something weird in my basement, i think this may be some sort of vent pipe.

why would they build a bathroom on a slab and not over the basement? i was thinking the part of the house with the bathroom (on the slab) was an addition to the house, but now i am thinking that could have been the original cottage and then they built on the rest of the house, which would explain the partial basement and lack of plumbing in it, but the well pump is located in the basement.

i hope i havent confused anybody

thanks
josh

deadonion said:

the sumppump in the basement discharges through a hose that is routed through the foundation wall, up through the slab under my sink, and into the same drain as my kitchen sink.

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That's not allowed in most areas and in general bad practice. (especially when you are on city sewer). The sump pump should run outside. BUT

deadonion said:

laundry, which empties into the sumppump,

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That needs to be fixed. I assume the laundry is in the basement. There are special pumps, similar to sumppumps called ejector pumps. Those are used for sewer water.

I guess it could be possible that you have an ejector pump but I doubt it if its hooked up with a hose like it is.

deadonion said:

why would they build a bathroom on a slab and not over the basement?

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Many houses are built on a slab only, even new construction.

deadonion said:

i need to know how to locate the main drain in my house

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You need to hire a drain scope service or rent one and DIY. This service will map out all the drain lines they have access to. They can tell the depth and also do a visual inspection. Also, the inspection service may be able to make sens of that pit by tracing the pipes leading in/out of it.

You can also do a free "call before you dig" at your city. The city will come out and mark all underground utilities outside your house. That will give you an idea where the sewer line enters your house at least.

Good luck