Washing machine closet

[Olf]

Because a washing machine/tumble drier becomes a killer the moment it walks into a British bathroom, we'll better keep it out. Kind of.

I carved out a 'closet' so the access is from the outside. Question: what constitutes a 'wall' the idea does work? Is a single layer of plasterboard enough?

It would be hard to join it at the corner, so I might slot in some batten and make the partition at the toilet side a double layer, but still no more than 50mm total thickness - is that any better?

[Stones]

18mm plywood, faced in WC with plasterboard so you can get a uniform finish? Ply would give you structure. You would need to confirm BC were happy though.