I have an addition on the back of my house. I have the floor, roof and sides up. But... the contractor has done such a poor job that it leaks and will not pass code. Needless to say he is no longer around. I have asked other contractors and they say about $8k to finish. Already put in over $6k it is 15x16. Is it feasible to take the walls down, leave the roof and brace it on 4 corners and make a large deck out of it

I need help please

I cannot imagine a professional worth his salt that wouldn't tear down the poor structure and start new. I am confident you have been told this because of warranty issues with material and labor.

Can you save by demolishing and scraping the old stuff (salvage some of it maybe?). Sorry you got stuck but I doubt you can expect to transform a bad job into a good one without paying more.

What have your contractors said about your idea? How many have you gotten estimates from? Try at least 3 to get a good idea.

As long at the place where it connects to your house isn't going to be prone to water damage and rot. Especially the edge of the roof. You don't say how both roofs run into each other.
and
Make sure your four corners, even if braced, can carry the load based on their size and distance from each other.
and
What will you do about the framing that isn't weather resistant?
and
What a shame! What about it besides leaking is shoddy? And what about it makes it leak?

Most things are "feasible". Whether practical and cost effective is another question. What are the floor joists made of? If not pressure treated, don't make a deck, it will be eaten up in a couple of years.