The word "foundation" conjures many images. To a banker, it is a financial organization. To a cosmetologist, it is a layer of make-up; and to a builder, it is the solid underpinning upon which a building rests. Just as there are some women who do not want to be seen without a foundation, you would not want your addition to be without its benefits either.
This is why the foundation of your addition must be built from a rugged and durable material, cheaply available, easy to work with and of the same chemical make-up as most cosmetics.
We walk, drive and live on cement, yet we take its benefits for granted. It provides a smooth surface for automobiles and bicycles. It joins bricks. It stifles the growth of unwanted plants. And, perhaps most importantly, it forms the surface of my patio, providing a place to bounce a basketball. (Just try playing the sport on grass. It becomes remarkably similar to football.)
Cement, in its pristine state, comes in the form of a heavy, brown bag filled with a gray, powdery dirt. There are two varieties: pre-mixed and un-mixed. I recommend un-mixed cement for the one reason that I ever recommend one thing over another: it's cheaper.
A certain amount of sand or gravel must be added to the un-mixed variety. Adding too little results in a waste of mortar, and adding too much leaves a crumbly mess vaguely reminiscent of Aunt Mabel's oatmeal-raisin cookies. (The ones she sends every year at Christmas.) The standard ratio of mortar to sand is approximately six shovels full of sand to one of mortar. Water is added until the mass achieves a workable consistency. However, before you can mix cement, you must get it home.