Question Add outer walls to imported house model

I'm really new to this and watch all the videos but 1 hour into a video I can see it isn't going to answer my question... I got a model of a living room which I added to my 3D scene. It works fine but when I go "outside" I can see thru the walls. I've constructed some rough walls that sit parallel to the inner walls. They can't sit at exactly the same position as the inner walls (I don't think) because I get z-fighting. So I space them out from the inner walls and now there is a gap along the edges of the cube that is formed.

Do I make these outer walls "bigger" to accommodate the inner room? That seems like a good idea but I can't seem to do it. Is there a formula where I adjust the transform and scale to some amount and they match up? Or am I doing this completely wrong? Should I create wall prefabs that have both sides? I get the feeling I'm trying to solve a problem that I created but rooms with inner and outer walls must be being created every day.

Help please

 

I could most probably still use some pointers but I did a good thing (I think). I created an empty object to contain each inner wall and the outer wall I created and now the positioning of them is considerably easier as they are relative to the parent object and of course I still have only 4 walls to position.