Hearthfire is the second DLC for the Elder Scrolls V: Skyrim and brings home ownership to a new level. With this addon, you can seak to own land in three different areas of Skyrim (Hjaalmarch, Falkreath, and the Pale) and on this land you can custom build your own house. Starting with a small base, you can add an entry way, cellar, and wings to your house, as well as a garden or blacksmithing area outdoors.
The DLC also adds the ability to adopt children but to do so, you must first have a room for them in your house. Building a house and then a wing with a bedroom will give the Orphanage caregiver enough reason to trust you. You can also change certain rooms in the houses you already own into bedrooms (such as the alchemy room in the Breezeway home in Whiterun).
After installing the DLC and entering a city, you'll be approached by a courier with letters for you, provided you either own a home already (like Breezeway) or have satisfied the requirements to become a Jarl in one of these holds. The first is a letter from one of the Jarls of those Holds that mentions a house up for ownership while the other letter will direct you to the Orphanage in Riften to adopt. Visit the Jarl's house in whichever city is mentioned (either Dawnstar, Morthol, or Falkreath) and talk to the Steward or Jarl there to buy the house. From there, head to your newly owned (and marked) property to start bulding.