There will be an opening and below the opening ( on kitchen side ) there will be a kitchen sink. In order words - somebody doing the dishes will be able to see the dining room. So my question is - what are pros of cons of such design ?

24 feet along the house. 15 feet across

The deck will be about 36 inches high

I live in NJ. The ceiling is currently flat. Also to clarify - I will be exposing ceiling "joists" and not beams

The solid lid in this is case is made by cutting existing hardwood floor which is nailed into 3/4 inch plywood. Of course, the bottom of the box such created to store ladder should be reinforced. But that could be done by nailing a 2x4 between two joists. Agreed it will require proper sealing otherwise spilled liquid will leak into basement. But not sure why insulation will be needed around this box. The 12 feet width shown in my diagram is actually width of room. But to reach a height of 8.5 feet, 10 feet should be enough. This can also be resolved by having a foldable ladder