I may have written about this before here. If so, this is an elaboration.
One of the things that was a feature of the house I remember growing up in the most, 10614 Tanager Trail in Brecksville, OH, was that part of the basement was unfinished, with a crawlspace and an oddly-shaped cellar that had big extensions in the way rooms are not normally big (human width but very long). My grandparents house had even more of this; my grandpa's office was initually in a large, semifurnished attic, always with hints that there were parts of house interiour that were not rooms that one might accidentally drop into if one went far enough into the slanted bits with the low roof off the main walkway.
I extended this into a set of myths about houses; the idea that there was a flipside to houses reachable by passages or doors or more awkward means, typically a long tunnel with its own set of rooms branching off in the space corrisponding to the house. The tunnel would usually be about the width of a driveway, sometimes a bit larger, and it'd usually have some minimal lighting of its own. In my dreams I'd sometimes use it as a shortcut, but always a nervous one because the place was imbued with the feeling that I was not meant to be there. Sometimes I would play or sit in the rooms corrisponding to my grandparents or my house; they felt like they belonged to the house in some way rather than the tunnel.
The tunnel itself continued far onward in two, sometimes four directions. In the dreams featuring it, I knew that in theory I could walk to the local city hall and if I spent some more time, I could actually walk between my grandparents and my parents house (in reality this would've been at least a day's hike, I think).
The tunnel space was spatially distorted. Beyond being parallel to normal space in a way that anyone using the entrances I remember would find the passage emptying out back into a normal room, the tunnels and rooms in the tunnelspace themselves didn't seem to occupy quite the same area; the rooms were far larger than they should've been, the equivalent of walking through a building and finding each of the rooms having internal dimensions such that they should overlap other rooms.
In tonight's dream, I was back in the Brecksville house, and my father had managed to replace a small corner of the basement where there had been a ledged window (which I sometimes used to exit the house IRL if I was in the mood to climb up) with a very large, very empty room that was in tunnel-roomspace but with a door linking it to realspace. I didn't see the full extent of the room to see if it actually had a door to proper tunnelspace; like a lot of the oddly-shaped rooms, it wrapped around.
(My father is a lawyer) I expressed a desire that were I still living at home, I'd like to move into the room, its ledged window turning into a nice skylight and the entire thing having the feel of a large greenhouse with a bare concrete floor (most of tunnelspace was unfinished). I then wondered if it might be neat to take a tent and sleep in tunnelspace, thinking almost immediately that maybe that was a bad idea as it was very remote and not a lot of people knew how to get there. He explained that at least in small towns tunnelspace was generally designated (but not used) as a military base, and trespassing on it, while practically never prosecuted, was taken very seriously. This is largely because a lot of people have trouble entering it, it has more depths than I have ever seen, and patrolling them (or even reaching some of them) was too much of a burden for the police.
I'm not sure if my dreams will keep this elaboration or not.
It's interesting that this dream didn't seem to have the neurotic anger I normally have towards my father in dreams, but I suppose this dream was set mostly in the context of my early childhood.
I was weirded out when my parents moved to their second house and it had closets that literally did extend into unfinished parts of the house; it'd look like a normal closet but one wall's absence just kept going and going and there was a corner and you'd be behind a different wall of the room. Great for storage, but pretty weird given what my dreams had been. Likewise, Philadelphia's extensive underground (content ranges from "mall" to "weird long tunnel with occasional large rats") feels at times like it might belong to that realm of dreams.