What is the HOUSE.PARTY achievement about?

The house in the background reminds me of the one in GROWTH. My guess is that you need to PERFECT the stage in which it appears. If I remember right that's 3-7. Can't confirm it though as I only have the Wii version.

So I'm posting a separate answer since you can't put images in comments.

According to an archived message on the steam forums, you only need to get a perfect in the subsection directly before the house appears:

I just got it. When you see the house catch every beat before the long trail where you have to stay in the middle.

The pattern is one of the hardest to 'get' but simple when you know what you have to do.

The pattern repeats after the 4 positions. And maybe you have to be in Hyper. I'm not sure about that. Good luck. ;)

After about a minute in 3-7, you'll hit a stretch where you have triplets of normal yellow beats to bat back. After this section is a group of the teal beats, which is your target for perfect-ing.

Directly after the teal beats, the House shows up (and should, theoretically, award the achievement)

Given that I suck at bit.trip beat, I need to leave it for someone else to confirm that this is the correct sub-string of beats to earn the achievement.