I built my house with 2×6 studs, giving me 50% more wall thickness and insulation. It costs a little more, but is one of those things that sets apart those crappy look-alike tract homes that I abhor, from the creative well-built solid construction, which is real building.
A house on a permanent foundation is considered a house for lending purposes. Otherwise, it is a mobile home and the lending guidelines are much tougher. The house we bought in 2000, before the fire, was a manufactured house on a permanent foundation. Nobody knew it was manufactured; the only way I was reminded was from the hollow sound in the floor when walking around, and the strips on the bathroom and kitchen walls to connect the drywall sheets. The newer homes don’t have those strips. BTW, I looked at a 2300 sq ft rental house outside of 4S Ranch ($3100/month), and I could tell right away when walking in it, that it was a manufactured home; the hollow sounds gave it away.