How much does building a house and all of the additions usually raise your smithing?

It raises you smithing because of all the items you have to make. For example.

If you start out with 15 in your smithing, by the time you finish one house, your smithing will be up to 50 +/- You have to have lots of iron ore/ingots. I mean lots of it. You will make nails, hinges, etc. This is how it raises your smithing. You also have to have the Hearthfire dlc.

It does end up having a good boost to your Smithing, though it's not like you're suddenly going to be able to craft super armor (lest your skill is already high).