Do terrain modifications build / stack?

[BloodandDust]

Examples:
1) ground is already icy (difficult terrain) - Wizard casts Sea of Thought, which makes terrain difficult... does this turn the ground into Greater Difficult Terrain?
2) ground is 'normal' - Wizard casts Mud pit, making the ground Difficult Terrain. Moments later he casts Oneiric Mire... does this turn the Difficult Terrain into Greater DIfficult Terrain?

Initial reaction was "sadly no, this would be an example of stacking penalties", which are generally not allowed.

However then I considered that terrain class is not a movement modifier like a circumstance or status penalty, so maybe they *could* stack... but did not see a rules reference right away.

[breithauptclan]

I would still say that it doesn't increase because of the Duplicate Effects general rule.

Difficult Terrain + Difficult Terrain = Difficult Terrain.

I vaguely remember reading a rules entry about this too. Something about casting a spell that causes icy terrain in an environment that is already icy. But now I can't find it.

[BloodandDust]

I have not found anything since either. I'll go with the default: difficult terrain + difficult terrain = difficult terrain. Ah well, thanks for considering it.