Suppose that the scene-setting narration includes "A fierce storm is howling!" And then player A declares that their PC says something to character B. Can A talk to B despite the noise of the storm?Now, if someone wants to walk across that frozen water--what does that mean? I'll argue that those decisions are, indeed, house rules.
I'm sure there are some RPGs that treat this via a discrete rules subsystem - and those might then require a ruling, or a "house rule", as to what sort of penalty the noise of the storm imposes to (say) a Listen check.
But in (say) Prince Valiant this does not require a house rule. The GM just applies the standard rule for resolution - the GM decides whether or not to call for a test, based on whether there is anything at stake in success or failure, and (if there is) assigning a difficulty for the test on Oration.
Walking on ice is the same - that would be a test on Agility.
MHRP uses a different, less task-oriented approach to resolution. What is at stake in the character walking on the ice? That will tell us what is mechanically going on (are they attacking, creating an asset, depleting a Scene Distinction, etc?). The ice then figures as an opposing factor in resolution, to be incorporated as in my reply just upthread.
And that is not house-ruling. That's just playing the game. The GM deciding how heavily to weight some opposing consideration, in the context of MHRP, is no more a house rule than a D&D GM deciding what direction the dragon breathes in. It's a moment of game play, not the creation of some binding norm.
If, next time, the GM thinks the presence of ice in the scene is not interesting, or is interesting for a different reason (eg that Pyro is going to melt it), they are at liberty to use any of the various mechanical resources at their disposal to make it a factor, or non-factor, in whatever way those mechanics permit.