Why sometimes it comes up with a "are you sure" type window and other times it just places the house. Sure is annoying when ur trying to judge for space what u can fit somewhere
I
think the placement without the "are you sure" message happens if you last placed a house less than a week before but it's been at least an hour since your account actually had a house on it. For example, you place a house on Monday. On Tuesday at 8 am, you demolish or trade away that house. Tuesday evening, you find another spot you like and use the housing tool. When you do so, the house is placed with no confirmation gump.
Again, I think that's how it works. However, it could instead be that the automatic placement happens for a week after you demolished or traded away the most recent house on the account. Hopefully someone who has more experience with this can demystify this!