I don't think we'll get a flooded basement, but I absolutely believe we'd get a bloody bathroom. It's iconic, and easy to pull off. The room would already be sprayed and you'd have an actress struggling in front of the sink, it would be a perfect distraction for a pennywise to pull back the shower curtain or have a boo hole opposite of her or to have her creepy dad pops out. They're going to want to feature all the kids in the movie without repeating or having a bunch in one scene, so it makes sense to get to see their individual moments and then a finale."Needed space" is dependent on what they do.
Let's look at the needed "space" of IT based on most transferable scenes. I'm calling these the "most transferable" because, after a year, these are the scenes that stick in my mind. I'm not an IT-fiend who knows everything about it. I'm a casual, and that's who these sorts of houses are built for.
Neibolt house façade (most likely)
Neibolt living room (medium sized room)
Neibolt bedroom (small room)
Neibolt basement (small room)
Flooded basement (small room)
Garage slide show (medium sized room)
Bloody Bathroom (small room)
Library (hallway)
Sewer tunnels (hallway)
The Altar/Spaceship/Whatever it is (big room)
Those are the scenes that matter in the film. Now, we'll likely only get the living room and basement of the Neibolt house scenes. I also don't really see the Bloody Bathroom scene because there's no actual scare in it. It's just a teenage girl scream. That leaves 7 strong scenes, and it's various forms can appear wherever. Now, Parade has had several mazes that was mostly small/medium size rooms and one massive scale room. Forsaken and 25 years immediately come to mind. They can also build a two story façade in there, which is what you need for the Neibolt house.
So, yeah. "Needed space."
Georgie scene would be hard to do, but then they pulled off the blood elevator in the Shining.
Mike and the Black Spot
Ben in the library
Bevs Bathroom
Maybe the flutist with Stan
The Leper somewhere
The Garage
Edie Richie and Bill somewhere in Niebolt house
A sewer scene or 2 with the bullies.
Finale with multiple It forms jumping out.
Not in that order but I think those are some points to hit.
I could honestly see It in a sprung tent. The Niebolt house as the facade. The only big room would be the finale room in the sewer and we've seen 2 story scenes in the tents before, but I imagine that the biggest IP would get the SS treatment.