You definitely don't have to remodel a room to build a decent Atmos system, and I don't think the placement specs have to be followed to the letter either.
I'm in a rental, so in-ceiling monitors (which some specs cite as a requirement) were never an option - instead, my heights (Paradigm Atom V3 bookshelf speakers) are mounted on top of Ikea KALLAX cabinets and pointed down. In-ceiling monitors are actually pretty uncommon in the mixing studios too, you tend to see more wall-mounted bookshelf-size monitors (like Genelec 8010s or Kali IN-5-Cs) angled down or even attached directly to the ceiling.
I also think there is some latitude with the placement of the height speakers - for example, lots of 7.1.4 diagrams (like the one below) show that the ceiling speakers are supposed to be positioned in a sort of 'smaller rectangle' within the 'larger rectangle' of your 7.1 floor array.
View attachment 93810
I can't figure why they designed it like this, because lots of Atmos mixes have elements suspended halfway between the front & front heights, rears & rear heights, sides & top middles, and so on. Wouldn't those 'phantom images' be clearer if the speakers were properly aligned?
So I set my room up with the front heights directly above the fronts and rear heights directly above the rears, and it sounds great - I can clearly hear stuff like the lead vocals in Sarah McLachlan's
Surfacing album or Katatonia's
Sky Void Of Stars hovering between the fronts & front heights.
Funny enough, Frank Filipetti made this same comment when I
interviewed him last year: