How do I install the washing machine in an attic bathroom?

You need to run separate plumbing to the location to accommodate the washing machine. Not the best placement in an attic, as if it ever overflows or gets a leak, it will cause water damage that can be costly to repair. I would seriously consider another location to place the washing machine.

Attics, by definition, do not have floor joists strong enough to hold a washing machine full of water & clothes. We’re talking 500-600 lbs.

If you’ve had a licensed architect or civil or structural engineer verify that your structure and your stair case will handle this load, in writing, then they will also have observed if your upstairs doorway is 2’-8” or wider, as no US washer will fit through a smaller door.

They can also visualize what you’re trying to do with the plumbing.

Ive seen big utility sinks hooked up to special washer hoses and that’s where the gray water drained. The lint clean out in that drain is essential. A bathroom sink won’t work.

Washing machines weigh a good amount, especially with water and clothes. Will the floor handle the weight? If it will, you will need to run hot and cold water lines and an outlet. Make sure the system can handle the extra amps.

Inspiration is always a good thing. We have homes built of brick, cinderblock, and SIPs in the States...are you going to route the waste water down your bathtub or shower? The shower pan may not hold the volume of water at the rate a washer pump can expel, the tub will. Or if you can get a waste line joined to the others, you should be good. When I brought our washer up from the cellar so we didn’t have to go up and down stairs, I fixed plywood to the hardwood and laid tile on top of the plywood. I also added some bracing to the joists in that area. I was concerned with the vibration of washing machines which can sometimes “walk” if they go out of level. We didn’t have a laundry room either. That was 20 years or so ago. It is holding the heavy front loaders with the drawers under them with no problem. Even my Grandmother had no laundry room at her house. Hers was in the basement with many more steps up and down. Carrying laundry up and down stairs is a pain and I applaud your thinking! I think laundry rooms started with the advent of dryers. But when I was in college, several decades ago, we had a small washer/dryer combo that probably didn’t weigh 100 pounds. It could wash two pair of jeans so we were doing laundry every day for four of us. And the dryer was worthless we usually hung everything outside on the line. I’m not sure if they still make them, and unless most of your clothes are sent to the cleaners, I don’t think they are practical. But if the floor joists have been beefed up for a tub and shower, it may very well hold a normal size washer. Just keep it level.