I must have read this paperback as a teenager, probably in the 1970s. I read it in German, but I'm pretty sure it was a translation. The cover may have featured a house, with the upper story detaching itself from the rest, but I'm not sure.
It's set in the US, I only remember a love interest that leads a young man (possibly a detective) to inquire into the rich Mexicans(?) who are living in a house together.
One of the residents collects Aztec art, I think. The upshot is that the upper floor of the house is in reality a spaceship with which they want to leave the Earth, taking the young man's love interest with them. The house featured also in the German title, but the rest is a blur.
The baddie is called something ending in "cotl" or "tetl".