Is William having a flight of whimsy near the start of chapter 38, or what? It's pretty clear that it's his idea, not Fanny's, but it seems quite unenterprising by his standards. It's not as if he isn't interested in women himself, either: remember his lament in chapter 25 about how the Gregorys turned up their noses at midshipmen. So, supposing that William gets the rapid career advancement he's musing about, does he seriously like the cottage idea?