Stacked units in the primary bed closet next to the tub wall. Huge closet, you will still have plenty of space in there.

I'd take some space from the Master BR closet and open it to the hall as Sabrina Alfin suggested. That enables the washer and dryer to be accessible by others without going into a private space like the Master BR.

IMO you will run up and down all the time doing laundry in either place I can't tell you how much I love my main floor laundry and I have many clients now where e did the same and they always tell me how convienient it is . One trip with the dirty clotehs one trip with the clean You spend most of the day on the mainflor why run up or down for every wash and dry cycle.

Make sure to contact your home owners insurance before doing this, it does add additional cost to policy.

Unless one has the washer dryer in an unfinished basement, any flooding by the units will cause issues since a bad flood can wreck an extensive area of the flooring.

The way to safeguard any kind of "watery" appliance is to eliminate the risk of flooding by having a shut off alarm system that trips when water is sensed outside the units; having a pan in place to hold any water so that it doesn't cause damage and to use extra heavy duty hoses.

FWIW this is common sense for both the dishwasher and a refrigerator that has a water line for ice or water dispenser.

I have these on my washer dryer, DW, refrigerator water line as well as my HVAC unit which is in the ceiling above my foyer.

A first floor laundry doesn't work for everyone. We had a 2nd floor laundry in a townhome where the bedrooms were on the 2nd floor and liked it a lot. For one of my clients with RA, we moved her laundry from the 1st to the 2nd floor and dramatically improved her quality of life by eliminating the need to lug laundry up and down stairs (the bedrooms were all on the 2nd floor.)

The potential for water damage should be considered irrespective of where the laundry is located. There are a number of automatic shutoff valves that sense a leak and shut water off. These can protect an entire home (e.g., Moen's Flo Valve) or a single use point (e.g., Watts Regulator's Intelliflow Auto shutoff for washing machines.)

At the head of the stairs?! That won’t present a problem when someone is running up the stairs while someone else is standing in front of the open dryer……

There are three 2nd floor bedrooms sharing a hall bath. I submit there are more complicated logistics dealt with on a daily basis in his household than will be required for accommodating less frequent laundry tasks in the event the laundry is at the top of the stairs.

I am more concerned about safety than logistics. I am sure that stems from the monthly visits we used to make to the emergency room! Bedroom door, head of stairs, and door on laundry equates to a Three Stooges comedy scene!

Our laundry room is on the main floor and we love it there.

Add it where the master closet backs up to the hallway. This would open up to the hallway and probably allow room for a side by side configuration with shelves or cabinets above.

You could also add a sliding door opening between the master closet and hallway, with the W/D actually in the master closet. That way you could hang up clothes directly from the dryer. Alternatively, you could make the laundry area a little bigger and put the units on the wall backing up to the middle bathroom. The opposite wall would have room for a hanging rack and a folding table. Otoh, if you don't want the laundry area opening into your closet, you could add a hanging rack like this on the laundry door. Please let us know what you decide. Photos of final result would be good too. :)

https://www.amazon.com/Clothes-Organizer-Clothing-Storage-Organization/dp/B07WC73LWY/ref=sr_1_8?keywords=Over+Door+Rack&qid=1663214136&sr=8-8