I have a 12' X 20" existing deck. I want to add another 5' to it. I don't want to dig a hole for 4x4's I want to put 2x6's at an angle 4' out to suport the 5' addition

You need to check into your local zoning office first.. these days many if not most communities require inspections of deck constrution for safety and this requires permits and plans. This is due to a rash of collapses where many people were injured or killed.

I do not know of a locality that will allow a 5 ft. cantilever without coming back into the main deck. This is not practical and is dangerous.

Cantilever? No way ! You dont have to dig. Why can't you buy the cement preformed tubes to place your posts in ?

In my area, deck construction where the deck is more than 3 feet high requires a permit. Get a copy of your jurisdiction's code, design or have a qualified rep design an extension that complies with the code -if your work requires a permit. As smoothy stated, deck failures are a very frequent cause of injuries or even deaths. And don't assume that your homeowner's insurance will cover you, if the work was not in accordance with code. And even decks less than 3 feet off the ground can cause serious (and very costly) injuries in certain circumstances.