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I'm in the process of closing on a house that has an unusually large crawlspace underneath the structure. The house is built on a relatively steep hillside and the frame of the house extends to the bottom of the hill below, giving an enclosed crawlspace.

Here's a diagram (with apologies for my MSPaint skills):

Elevation view of the house

The crawlspace has approximately 4.9m (16.2 feet) of space between the concrete floor and the bottom of the timber frame supporting the ground floor. There are no stilts or other support structures in the main part of the crawlspace though there are in the other half.

Here's a photo on the inside:

Photo showing the crawlspace

I would like to have this basement finished by adding an internal staircase from the ground floor down to this area. I would like to split this space into two floors. I understand that each floor must have at least 7 feet of headroom, and I assume you'd want a foot of space between the two floors and plus another foot to account for space from the ceiling below the ground floor and the bottom floor's carpeting. That's 7+7+1+1 == 16 feet. So far so good, right?

During the house inspection, I asked the inspector how possible it would be to do the conversion and he said it would be impossible as you need at least 10 feet per floor, so the house is 4 feet too short to convert the crawlspace into two floors. He was also pessimistic about installing rooms (such as a toilet room or a small bedroom) on the area currently covered in black tarp. He said that converting the main area of the crawlspace (with the flat floor) is definitely possible, but I'd probably only be able to have a smaller area (with a 5 or 6-foot high ceiling, maybe) underneath the finished room suitable only as a storage area.

With references to suitable building regs and whatnot, is my inspector correct in saying I can only have 1 fully-finished floor, or can I still have two finished floors/storeys in the space?

This is in King County, WA. If that helps.

  • You may not be able to make it two legal floors, but I doubt there would be anything forbidding your. building a loft into part or all of that space to get a more even division of the space. As far as the slope goes, remember that if the folks who built the house could have excavated into that to produce a full basement, they probably would have done so. – keshlam May 25 '15 at 5:06
  • @keshlam the other new houses on the same development all have finished day-basements. My agent reckoned that they planned to do the same to mine, but ran out of time and wanted to sell sooner than wait another month or two to have the basement finished. – Dai May 25 '15 at 6:37
  • I could see really nice loft storage, storage above the hillside, and a run of stairs coming down your dirt hill... Is there a particular number/purpose of rooms you're trying to gain, or is this just a general, "what can I squeeze out of this space" kind of question? – Aloysius Defenestrate May 25 '15 at 14:40
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    @dai: As likely, or more likely, that they hit rock they didn't want to try to drill/blast through. .. – keshlam May 25 '15 at 15:15
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You have two issues to contend with.

First is that 16 feet is just a bit too tight for two floors. Sure, we usually use 12 inch beams but that's just the structure - you need an actual floor on top and usually a ceiling below. Even if we ignore building codes we will still produce two rather cramped levels.

Second is what's actually holding the other end of the house up. If they built it on the hill, then that pile of dirt is rather important - you can't just shovel it out without adding house-grade support as you go. If it's filler behind a wall there is / will be a lot of pressure on the other side that you have to account for.

The black tarp is a water / erosion / insect barrier, and a rather cheap one at that. I would do spray-on concrete over the dirt, work in some levelled 4x4 beams when the concrete is being done, and develop the crawl space as a high-ceiling recreation room with lots of storage over the hill (resting on the 4x4s).

  • What's that dark spot in the corner? If there is any kind of moisture penetrating the basement, fix that before finishing it. I think I would make one high ceiling finished floor and a crawlspace for storage and running wires. I'd probably leave the tarped area alone, maybe just spray concrete on it. Also bear in mind that you make not recover the cost for building below grade. – BrianK May 26 '15 at 1:59

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