I would guess that you are looking at a huge capital outlay here. A lot of it depends on location the elevation of the sewer line going out of your house.

Is there a bathroom down there? Or would the tenant use one upstairs?

There is a bathroom down there, yes.

How huge is huge? Hundreds? Thousands?

Well, what would the drain drain? Is this something that you legally need to do?

It all depends on what's there, and what you can tap into.

I'm cautiously optimistic based on the implication that the plumber only has to find something that's already there. I thought that having a drain in the basement floor was a basic building code issue. Even the oldest basements I've been into seem to have them. Do you think some knucklehead covered up the drain with tiles?

Yes, I think some knucklehead covered up the drain with tiles. After all, in the very next room some brainiac covered up a working electrical outlet with wallpaper. I've also got an idea it's code to have a drain and that there must be one there somewhere.

Last March I had a burst pipe and a flooded basement. If I had not been home the whole basement would have filled right up. I figure not having a drain is just asking to have that happen sometime.

Well, did the same knucklehead level the floor? I bet they didn't. Find the lowest spot on the floor and that's probably where it is. Just hope they didn't fill in the drain hole with cement or something dopey like that.

Good idea. No, the floor is not level. When it flooded the water seemed to pool in the middle, and also when I was painting my step stool wouldn't sit properly on the floor.

I do hate the thought that the tile floor will have to be torn up and "patched" around the area where the drainpipe is, but hopefully I can find some tiles that are a fairly good match.