It seems this is a big stumbling block many people encounter - how to connect two rooms with a hallway. There really isn't anything difficult about the process. Its best done "piece by piece". Follow the steps below.
Step 1 - layout the floor plan
You want to know what the floorplan for the area your're working with is. We'll assume that you're building the rooms as well as the hallway here.
Step 2 - build the walls
Next, build up your walls. The picture below is quite simple. It would be a touch more complex if the room walls and hallway walls were of different height.
Step 3 - smack a ceiling on that sucker
Um, put on the ceiling.
Ok, so what do we have now?
The simple answer is, not much, really. You've got two boxy rooms connected by a plain boxy hallway. Hopefully what you do have now is an understanding of the concept of how to connect two rooms with a hallway. You should be able to apply this to new rooms or rooms you've already created (by deleting or clipping away holes in your walls and smacking together a quick hallway).
Just think of it as: putting a hole in each room then putting down a floor, ceiling, and two walls between them.
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