Bit.Trip Beat now has two new achievements: SUN.BATHING and HOUSE.PARTY.
Now, the Steam Summer Camp initiative makes it clear that SUN.BATHING is about perfecting 1-1, but HOUSE.PARTY remains a mystery.
How can you unlock it?
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:
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.
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.