Intermediate user here!

I’m making a hardsurface model which has a lot of intricate detailed mesh pieces which i have separated individually but is still one object. but when I add the sub surf modifier I have gaps in the edges which defeats the illusion as to they are all one mesh! how do you go about fixing this so we can make complex objects in different pieces all combined as one.

Help would be great!

An image/blend file of the problem would be helpful. If you add a bevel modifier before the sub surf modifier it and maybe put the segments up a couple that should fix the problem. It depends on the exact situation though.

Well, different meshes even in the same object will be subdivided separately. That’s the way it is done.
You can select the nonmanifold edges and some supporting edges and crease them (shift-e), the weight can be adjusted.