The first upgrade to your house will unlock this achievement and is required to be able to have a wife/husband move in with you.
It will cost 10'000 gold and 450 wood.
You buy it from Robin at the carpenters house north of the town and the community center.
It adds a kitchen where you can cook all the recipes you learn and a fridge you can store food in. If you have all the ingredients for a recipe in your fridge you can make it using the kitchen counter/stove, no need to put everything in your inventory.
It also upgrades your bed to a double.