Mordenkainen's Magnificent Mansion
You conjure an extradimensional dwelling in range that lasts for the duration
Does this extradimensional space count as a separate plane for effects dependent on the plane a player is on. For example, take Sending.
[...] if the target is on a different plane than you, there is a 5 percent chance that the message doesn't arrive.
What about the various planar effects outlined in the Dungeon Master's Guide?
(DMG p. 47)
Creatures on the Astral Plane don't age or suffer from hunger or thirst
(DMG p.61)
As an action, a creature on Limbo can make an Intelligence check to mentally move an object on the plane that it can see within 30 feet of it.
