You have to approach it from the side, and your cursor cannot touch the edge of raised section. It's hard to put into words, really, but basically you want to take a long and flat view in order to pull it off.
"...freedom isn't a commodity to compromise." -- Captain America, New Avengers #21
Guide to Base Teleporters
Oook!
To sink items under a floor you need to have a lower floor section available on which to slide the item you're placing. You will want to set your camera angle so you're looking at the higher floor section from the lower floor section. Rotate the camera up/down so that you can hardly see the upper floor. Pick your item and place it on the lower floor section, then slide it under the upper floor section. Once it is placed, you can rotate the camera to view from under the room you're working on & adjust the location of the item.
I believe the problem you're having with the item popping up to the top floor section is when your cursor crosses the line between the 2 floor sections, it pops the item up to the higher floor. That can easily be fixed by adjusting your up/down camera angle while you're placing your items.
I have also found a way to sink items in the floor without having a lower floor section open. You can actually place an item under the floors from the side of a room. It's a little trickier than the dual floor sections but works if you want a room without a sunken floor section or items sunk in every section.
As for the chaise lounges by the pool, that was a little tricky as the operation table is shorter than the floor sections. For those I had to sink a safe under the floor, get it in the right position, then sink the operation table and slide up up the side of the safe. I takes a little playing with, but once you get it, you got it.
Note: You can't place an item under the lowest floor section.
I hope this is helpfull.
I know I've seen posts about how to do this, but I've been hunting for the last hour or so, and can't find anything on how it's done. How do you place items under floors? I saw a cool pool room where the builder was using laboratory tables as chaise lounges and they were partially sunken into the floor. But when I try, I can only get them about halfway to where I want them and then they spring back up to the top of the raised floor.