I am trying to figure out if I can add an ICF garage with a flat roof which would be used as a patio and entrance for the rear door of the house.  My yard is sloped and currently I have a tandem garage thats 16' x 42' which runs the length of the house.  I am thinking about adding a garage thats 22 to 24' wide that would run the entire length of my foundation 42'.  My lot slopes so that the entrance is already level with my existing garage floor and the other end in about 6 foot underground. I realize that the garage roof would only be about 7'6' in height it will still allow me storage for 4 cars.  A couple of questions is could you attach the new garage roof to the existing foundation wall so only 3 new walls would be required? Also, how can you make the roof water tight and have a surface you can use as a patio for the grill and furniture? I am thinking that the new garage floor could be dropped an additional foot to allow for a greater interior height. My lot does not allow a lot of room for a free standing garage in the back yard. I live in CT.

Any thing is possible if you have an engineer and money.
But I am sure that an engineer will not connect two structures together.

That could easily be built with any of the foam systems, such as lite deck, as well as the composite systems such as Hambro etc.