Bashir, to get an effective guidance on your question, it would be helpful to provide a floor plan showing plumbing, windows/doors, existing layout and to describe a bit about your vision. It can also be helpful to provide a plan that shows relationship to other rooms. Pending that information, I suggest considering the size of the sink and shower, storage needs, how many users and whether you prefer built-in storage or storage units (as in moveable furniture/shelving). A trick I have used is to build in a pull-out pantry that runs the depth of the shower--dramatically increasing storage in what would otherwise be unused space. Good luck in your planning!

re the floor, what about garage matting tiles, as in photo attached, but lay them in a groovier pattern, perhaps a chevron or 60's pattern, if leaning towards MCM. It may not be any dearer than the cost after getting a tiler in, if laying tiles? Also, you can take them out to clean them if need to and change the look whenever you feel the need!

I adore my 'garden room', it faces north to catch the glorious winter sun (and my plants love that!) and all of the fly screened sliding glass doors and big windows can be closed tight to keep out the winter westerly's or opened wide to catch beautiful breezes in the warmer months. Spring and summer are my favourites....just a step outside any one of the sliding doors and I am in my garden....I love it :)

my suggestion would be a solid brick wall with full width lowlight double glazed sound proof sliding windows (about 750 of the floor and only about 600 mm tall so that you can look at your gorgeous palms from a seated position without seeing above the fence, also add matching sliding glass doors to completely enclose the space and paint the new wall a dark colour so the focus is on the view....and, if you don't already have some roof ventilation a ceiling fan and roof vent will make the space more comfortable when you need to keep it all closed up for privacy and the side fence needs to be tightened up to cover the gaps and also paint the whole fence in the darkest colour to highlight the palms...sound is like air and water and can squeeze through the smallest gaps so all this effort and expense will be a great investment in your peace and privacy