I think you unlock it either after a certain amount of days pass or during the story quests, and you can get the planks from Logan's Sawmill in the first town, Bernard have the item needed for the Garden in the same town as well.
upgrading your base I don't think does much until your attacking by bandits in your home. Then you can reinforce the hut with !it's defence and can't get raided while asleep when the bandits hate you
