I have two rooms in my 1500 Sq ft 3 bd house. They still have carpet from before I moved in and honestly it never smelled that bad, a little musty, a little old, but the carpet looks fine, and I was happy for a long time. I had someone else stay there with a couple dogs for almost 2 weeks and they said they couldn't handle the smell.
Background: I have a cat, he has had some urine issues but usually on the bed or couch. Pretty much caught it all and cleaned it. Bad smell doesn't smell like cat urine to me. It's a old house, built '66, wondering if the subfloor needs replacing.
Fallout: I've since done some deep cleans and am now dealing with a worse smell than ever before. It's the carpet



