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I’m looking into a new construction home that lacks a floor drain in the laundry room. As I’m not a plumber, I’d like to understand how trivial or not trivial it is to add this, in case the builder pushes back on the ask.

It seems like it’s real simple as it’d be a separate drain pipe that, at some point, connects to the rest of the drainage system. Is it that easy or are there complications?

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    Have you looked underneath to how easy the access is? Digging through concrete to install the pipe with half a degree of play in the slope is very different than having 3 feet of crawlspace to route the new pipe. – ratchet freak Sep 26 '19 at 13:19
  • @ratchetfreak - the home has a crawlspace; it’s not on a slab – Craig Sep 26 '19 at 13:22
  • Need to know more about the construction of the house (slab, crawlspace, basement, etc). Does this level of the house already have showers and/or bathtubs? In some basements, the drainage system exits the house above the basement floor level. – Mattman944 Sep 26 '19 at 13:26
  • @Mattman944 The house has a crawlspace. The laundry room itself already includes a recessed washer box and a sink. The floor of the house has two bathrooms; one bath is adjacent to the laundry room although the laundry room plumbing is not on the adjacent wall. – Craig Sep 26 '19 at 13:40
  • The question is too broad. If you'd like to revise to describe your situation and ask something more specific, go ahead. As it is you'd need a booklet written to cover all the implications of such a project. – isherwood Sep 26 '19 at 14:00
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You need proper slope on the pipe—depending on the location of the drain and the location of the stack this may be difficult. At least with a crawlspace assuming sewer level is lower than the floor (ceiling assembly of the crawlspace) you should be able to make a giant mess of pipes that just hang and not have to touch framing. A builder will probably have to pull the plumber in to make the change. Even simple changes are going to be relatively expensive. Probably not as much as if you hired a licensed plumber to do it later but I wouldn't expect it to be much less.

The floor where the drain is located also needs to be sloped appropriately for the drain to have the best effect. Unless the floor drain hooks up to a pipe before a trap, it will need its own trap. This trap will be subject to drying out since it won't be in regular use and you'll have to periodically dump water down to prevent sewer gases from coming up unless you have a self priming water valve that occasionally drips water into it.

There is also a critical arm length for the pipe from the trap exit to the vent to prevent siphoning but presumably you'll be in range since the laundry machine also needs to be within this range for its trap.

I went with a flood safe solenoid and a water sensor that turns off the water to my water heater if the sensor detects water on the floor. These exist for laundry machines as well. You can even get a setup that texts that water has been detected and the water to the machine has been shut off. This might be cheaper than the floor drain route—though in principle I like passive insurance. I also go with a washer pan under the washing machine; you could put the sensor in the pan.

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Builders have to have plans approved before construction and making changes to certain design elements would mean the builder would have to go back to get plans redrawn/amended and in some cases approved again. Some municipalities/builders can be quite lax or strict, but it might explain why some builders refuse or charge ridiculous amounts for what might seem like a small changes.

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