How does one attach a addition to a existing house at the back ?

Simple - you just nail it on.

Seriously, you need so much more information here. What's the framing of the existing house, what's in the walls that need to be removed - electrical, plumbing, hvac? Not to mention that you need some sort of a foundation or slab for the addition, framing, sheathing, roofing, siding, and a host of other issues.

You need to first talk with an engineer or architect, then your building department, finally a builder who can do this for you.

Agreed, we need to know about the existing structure and foundation and what you plan for the addition. If you are on a slb you don't actually make a mechanical connection of the slabs, they butt each other but float each on their own.