Lowering a room...digging into foundation?
The nature of what you are proposing here is really beyond what you can get proper response to from a group of folks on the internet. You really need to subscribe the services of a structural engineer in your area before you "break into" anything. They will be way more experienced to evaluate things like:


Existing foundation structure
Building construction technique
Building regulations and codes in your local
Whether the existing structure has on grade concrete floors or above grade wood floors.
Access to public building permit records for your abode
Comprehend the ground type on your lot relative to local conditions
Familiar with climate zone and how that applies to your existing house.