Grand Design 398M half bath and washer/dryer

A little bit of storage
A little bit of storage

We full time travel in a 2021 Grand Design Momentum 398M-R with 6 kids. We also have the optional half bath in our garage with a Splendide stackable washer and dryer. A question that we get (a lot) is “What are the exact measurements of the half bath?”. This is because a) Grand Design doesn’t really give any details about it and b) very few dealers have one in stock so it’s almost always a custom order.

The question that usually follows the first is “Is it worth it? Would you get it again?”.

Is it worth it?

I’ll answer the 2nd question first. The answer is, it’s a mixed bag. There are things we like about it, and things that are truly cumbersome and not well thought out at all. Some days we wouldn’t, and some days we would.


  • It gives us a second 55 gallon black tank
  • In an era of Covid FUD that leads to public restrooms being closed, we have a second bathroom in “urgent situations”
  • 2 kids can get ready for bed at the same time


  • It’s small. Super small. You’ll see the pictures in the next section, but my shoulders touch the walls. It’s strictly for the kids only.
  • The second black tank is shared with the kitchen sink and therefore the washing machine. We found that out the exciting way (black tank “water” can come up through the kitchen sink when doing more than three loads of laundry). If you don’t have the half bath the kitchen sink is connected via the gray tank like you would expect. Grand Design engineers found it more convenient to drop the extra plumbing leaving this unsanitary mess if you don’t manage the rear black tank like a hawk. We empty the tank before we start the morning laundry and again after the second load. That works well unless we do an excessive amount of dishes.
  • There is no vent in the bathroom. The window opens a little which helps with fumigation, but it isn’t really enough.
  • The bathroom door is located right next to the washing machine hookups. That leads to its own host of secondary problems.

Secondary Cons

  • There are 11 inches between the door and the washing machine which leads to
  • The bathroom door can only open halfway if you have a washing machine
  • The only (propane furnace) heater vent for the entire garage area is located directly behind the washing machine and
  • If you have a stackable washer/dryer the dryer will both block all the controls for lights and awnings in the garage and will require the TV be taken down (we moved ours to the master bedroom)

What are the measurement of the half bath?

We have a gallery of pictures that details the setup and dimensions. We also have a picture of how we secure the washer and dryer in this post.

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