Everyone knows gnomes live in mushroom houses. (gnomelover.com)
Players can also spend their hard - earned star coins to buy a powerup from various mushroom houses which also lets them save, but they very well might not want to spend their coins. (sirlin.net)
These are the levels in question; by getting at least the specified number of Coins, you can find the special White Mushroom House, accessible from the overworld. (gamerankings.com)