I have recently answered a post on Arqade (gaming.stackexchange.com) about if a village can be expanded (click here to see the post). I decided to make a blog about this for anyone that wanted to know:

The Village House

A building is a village house if it has a wooden door, an "inside" and an "outside". The "inside" needs a light level of less than 15 and should not have a direct view of the sky, while the outside needs a light level of 15 and a direct view of the sky. The "inside" is the side which has more "roof" blocks than the other side of the door, with the "roof" blocks being within a five block range from the door. A house does not have any requirements other than those three. An example of the smallest house is something like this:

Breeding of Villagers

Villagers will try to breed to keep the population above 35% of the total amount of houses. A villager must become "willing" in order to breed. If you want to make villagers willing, you can add a farm, or give the villager either 3 bread items, 12 carrots, 12 potatoes or 12 beetroots. Villagers with too much food will throw their food to other villagers, in order to make them willing too. You can also make them willing by trading with them. When trading a new item with them for the first time, they will become willing. They also have a 1/5 chance to become willing once you trade with them. If they become willing after you trade with them, green "sparkly" particles will appear above them. These particles are called "happyVillager".

Warning in Regard to Expansion

Note that a Zombie Siege can occur if a player is in the village and if the number of houses is above 10 or the number of villagers is above 20.