Basement Expansion Question?

[Legendary]

I am planning to build next year and I need some help with this concept. While on the golf course driving around and seeing homes with a huge deck on the back with nothing under it, it looked like a waste of good possible living space. My first thought was, I spend most of my time at home playing pool and watching tv why not extend the basement 20 feet in the back of the house and placing the deck on top of the extension to add 800 to 1000 sqft to the basement. Move the family room downstairs, add a bedroom, pool room, exercise room, play room for the kids, or storage space. Almost like having a two story house, but with the main area under ground.

My question is; would this be an expensive alteration? And would it be possible to extend under the front of the house as well to add more space? If so what costs could be associated?

.My Plans so far would be to build a open ranch style home w/1500sqft. I would finish the basement over time.