Turns out the double toilet thing in our 1950s place was actually a thing

I had a similar situation a while back when I was working on a fixer-upper in the suburbs. We had a separate fixture for the utility room that nobody used for years, but the main house already had its primary bathroom setup. We had the space for a second one, so we just wanted to see if it was worth the extra labor for a guest toilet. I ended up installing a standard two-piece model, but the flange wasn't quite right for the old cast iron drain, which made the seal tricky when I set it. It turned out to be a big hassle getting the connection sealed properly because the old stack was slightly off center. I ended up having to cut a small section of the pipe to make it flush, and then used a high-grade pipe dope around the joint to make sure there would be no leaks down the line. The drain needed to be perfectly aligned for that job.

Even though it was an extra project, having the separate unit made guests much more comfortable during renovations. It felt better than having a crowded main bathroom. Just be ready for some old pipe surprises when you are planning that installation.

In my last house, I faced a similar dilemma with a seldom used powder room off the main hall. It was in a late 1960s ranch, and the layout left us with a decent footprint but a tricky drain connection that ran into a crawl space. The old setup had a very short stub leading to the main soil pipe, and if we just dropped in a modern toilet, the required trap depth felt impossible to acheeve without raising the subfloor. I ended up consulting with a local plumber just to understand the proper fall, not needing full advice, just guidance on the minimum required depth for a solid seal. He mentioned that with the specific diameter of the sewer line we had under the slab, we needed to ensure the base of the fixture sat at least two inches above the subfloor level to prevent siphonage issues with the older system. Instead of going for a full two-piece model like someone mentioned earlier, I opted for a one-piece unit and used a diffrent flange system. It was a bit more work to cut the tile around it, but the resulting sealed connection made a huge difference in how quiet the unit was, expecially when flushing. It’s a tight squeeze in there, but getting the pipe alignment correct was the biggest hurdle to getting the install done right.

Interesting to hear a firsthand account on the builder motivation, because I dug through some old trade catalogs a while back when my uncle's place turned out to have a similar setup, and the mid-century ads really did pitch extra fixtures in finished basements as a modern convenience feature. His pair dates to about 1953 and the second one sits barely two feet from the laundry tubs, wich always struck me as tight spacing til I realized nobody expected guests down there. One thing I'd suggest keeping an eye on: a spare toilet that just sits will let the trap water evaporate, so flush it every few weeks or you'll get sewer gas drifting up.