The poem takes place in a house near a lake, probably out in the country somewhere. There are trees around, and it's probably a pretty nice place to visit when the weather's good. Too bad the weather's so crummy on the night the poem takes place. It's raining and so windy that the speaker imagines that the wind is consciously trying to break down trees out of "spite" (line 3).
The speaker doesn't tell us much about what the inside of the house looks like. There's no fire in the "grate" until Porphyria arrives, so the house is probably pretty cold. If there's no fire, there must not be any servants (most middle class Victorians kept at least one servant), so the speaker might be relatively poor. After all, the house is described as a "cottage" (line 9). Porphyria sure does a lot to cheer up the inside of the house, though! The fire makes everything all cozy. It doesn't seem all that bad – a nice cozy cottage with a bright fire on a rainy night. Seems like the perfect time and place to curl up with your significant other and cuddle by the fire, right? Sure, until the speaker decides that it's also the perfect time and place to strangle Porphyria to death with her own hair.