Do spells and abilities that are limited to the 'same plane'

work between different layers within an outer plane? I mean, there's no guarantee that spells like scrying work in the first place as a lot of outer planar powers will block divinations as a matter of course, but I'm curious about the default assumption.

I would think that layers would be considered different planes, but that's DM's call.

I also don't spend a lot of time thinking or reading about the default D&D cosmology, so there could be well-defined answer I'm unaware of. 

If you can theoretically get from one to another by physically traversing space, they're likely the same plane. If magic has to be used, probably not.

Quote from jl8e >>

 

If you can theoretically get from one to another by physically traversing space, they're likely the same plane. If magic has to be used, probably not.

That's how I'd rule it as well. What makes it another plane is it not being possible to just walk/swim/fly there.

I’d say it’s the same plane. In the DMG section on cosmology, it says (emphasis mine):

“Most Outer Planes include a number of distinct realms. These environments are often imagined as a stack of related parts of the same plane, so travelers refer to them as layers.”

So layers is just a shorthand way to describe different areas of a plane, not different planes. “Imagined as a stack” to me says it’s mortal minds fitting the idea of fungible and infinite spaces into something more comprehensible, but in the end it’s still all on one plane.