There are two available house upgrades in the game. You start out with the basic Log Cabin, then you can upgrade to the Deluxe Log Cabin, and then the Super Deluxe Log Cabin. To get married, you must upgrade your house at least once, and to have children after marriage, you must upgrade your house twice.
Deluxe Log Cabin
Wood: 250 pieces
Cost: 5,000 G
This upgrade is needed if you want to get married.
Doing this before the end of Summer in Year One will give you a clock.
Super Deluxe Log Cabin
Wood: 500 pieces
Cost: 10,000 G
This upgrade is needed if you want to have children.