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I live in a two story house build in the 60s. The lower floor(half buried) is only under half of the ground floor so I think there is a possibility that there is an extra room beside a wall in the lower floor. When I knock on the wall it emits a hollow sound.

Any ideas on how I could find out if there is a room behind the wall without drilling through it or knocking it down?

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    Are there paintings of people on the wall with their eyes cut out? – DMoore Oct 18 '14 at 20:33
  • What kind of wall is there: Concrete? Drywall finished lumber framing? Plywood? – wallyk Oct 18 '14 at 21:18
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    drywall walls will sound hollow if you aren't knocking directly on the studs. doesn't mean that there's a void on the other side. your best bet is to open the wall up and see what's going on. drywall can be replaced for reasonably cheap, and having a secret room is immeasurably fun (unless it holds corpses etc., of course) – alt Oct 18 '14 at 22:32
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    No crawlspace at all on the mystery side? There should at least be vents you can shine a flashlight through on the outside of the house. – Wayfaring Stranger Oct 18 '14 at 23:54
  • It might even be a slab on grade on the single floor side.... – Jack Oct 19 '14 at 4:06
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Sure there are other ways, you could spend tens of thousands of dollars to have it x-rayed (they do this in commercial buildings), but realistically, drilling a small hole and using an inspection camera is the simplest and cheapest route.

You might also try contacting your cities building department to see if they have any plans on-file - this might show you what is there.

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You could analyze pipes and wiring, looking for things unaccounted for in the rest of the house. But the only way to know for sure is if you open a hole and have a look.

If the lower floor is "half-underground" it would be fairly unusual to have no room there if the ground is level and "halfway" all around. If the "mystery room" side is uphill and fully or near fully underground, while the other side is fully or near fully exposed on the downhill side (thus still allowing for a description of "half-bured") you are probably going to be disappointed in your "mystery room", as it is very common to have a step-wise foundation in that case, and the area will very likely be full of dirt, probably with drywall over studs over concrete wall, from the description.

I managed to find a "mystery cubbyhole" in my abode once by noting that measurements didn't add up. Nothing exciting in there, just a "dead space" (per the thinking of builders who didn't believe in storage space, I guess) covered in drywall and closed off. After scratching my head a lot and triple-checking how the measurements didn't add up, I cut out an access hole in the drywall between two studs, and there it was. I also lived for a while in a house with no mystery - there was a narrow, accessible space behind the uphill wall of the lowest room, and the wall behind the wall was raw rock that the foundation had been quarried into, which leaked out water when it rained. The non-mystery space was there to divert the water into a drain.

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    Good answer. I have a small room in my basement that was closed off by 3' thick stone foundation (old house), now accessible from a removable panel. No buried treasure inside, but plenty of sterile dirt :P – J. Musser Oct 19 '14 at 0:59
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You could measure the length of the inside floor, wall to wall, and compare it to the length of the exterior, right? Just keep in mind the 2x4 walls and subtract that from the exterior length.

If there is no real variance, then you no longer need to be curious, but...If you have a couple foot variance inside, than cutting a hole would make sense then. Yea?

Also, you could cover a hole with a vent cover (looks like these people did that to cover a hole...see the light).

vent

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    You might get quite the draft out of a hole like that... – J. Musser Oct 19 '14 at 1:12
  • IF it goes to the outside. if it is truly a room, maybe not so drafty? – Carl B Oct 19 '14 at 2:06
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    An closed-in or built-off section of the house is likely to be unheated, so cooler than the other side, especially in a half-dug basement in winter. – J. Musser Oct 19 '14 at 2:09
  • @J.Musser Not everyone lives in Canada or upper-state NY were all those home improvement shows were made! I would love to get geocooling from being below ground. – JDługosz Oct 19 '14 at 8:16
  • @jdlugosz Yes, good point. My comment only applies to a large percentage of the world. – J. Musser Oct 19 '14 at 11:46
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I created a detailed plan drawing of my upstairs and found two places that didn't add up. I had brief hopes for more closet space, but I think it's where the ductwork crosses from the attic to the lower floor.

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