Rommel,
There are many factors to consider for your project. First thing I always do when considering a project is head down to the city with a ton of questions and ask away. This will get you a clear understanding of what you can and cannot do. This information will be first hand and accurate. I build all over the country and cannot emphasize enough how much the rules change between jurisdictions.
A few things to consider. If you choose to take the frame down and start from scratch with the new structure will you reuse the foundation? In my area all foundations need to be engineered, depending on where in the valley we build the foundation costs could be $30,000 to $100,000 K.
Saving the foundation will add extra cost to your demo because you will need to be carefull.
If you are going to take this on I would first have two scenarios drawn. A remodel and demo and new.
It would seem unless you are changing the footprint using exterior walls would be a way to go.
There are so many factors to look at it is tough to advise without knowing the extent of the whole project as @Sam LLoyd said.