our home is 26 feet wide and 50 feet long i would like to build a room 20 x 40 feet under the house never touching the currant foundation staying about 3 feet from the sides. how do i go about the waterproofing.

How deep are your existing footings? If they are shallow the 3' will not be enough unless you chnage your plans a bit.

This is not a DIY project, so contact a contractor that does underpinning or house moving, since there is a lot to consider on a job like you propose. They will probably find an engineer they usually work with.

You'll find that adding space in that manner will be more expensive than adding space above ground, or even adding a second story, if you don't already have one, and all you will achieve will be substandard space that will add little or nothing to the value of your property.