Heidi Klum and BF -- Couples House Hunting ... Scoping Katy Perry's Mansion

Heidi Klum and her rebound guy are getting super serious ... like, buying a house together serious ... and one of the places they're checking out is Katy Perry's L.A. mansion, TMZ has learned.

Heidi and her boyfriend Martin Kristen have been house hunting all over L.A. recently -- and sources tell us they're looking for a "family home" that has enough space for the couple and Heidi's 4 kids.

We're told Klum knows exactly what she wants -- she wants to be east of the 405 Freeway and needs 4 bedrooms for the kids that are ALL located on the same floor as the master.

Enter Katy Perry -- who still owns that incredible mansion in the Hollywood Hills that she purchased in 2012 with then-husband Russell Brand ... which has everything Klum is looking for.

Klum and Kristen biked over to the place last week to check out the 7 bedroom, 11 bathroom estate ... and we're told they were impressed.

It could be a perfect situation for Katy -- who recently reduced the asking price of her sprawling dream palace to $6.495 million hoping to motivate buyers.

Whoever snatches up the place is gonna get a helluva property -- it's like a real-life castle, complete with a lagoon-style pool and views to die for.