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I have recently bought a piece of land to build a country cottage and perhaps eventually move there permanently. The terrain is sloped with only a few flat spots. The former owners dug a large rectangular hole in one of these flat spots, with about 1,5m deep, with the intention to have a pond, but never completed it. The hole has almost the same size I would plan to build a wood house, and at the right spot.

Since I plan to keep the available flat spots as they are, to have yard, and since I don't have any clever plan for the hole, I was wondering:

Would there be any problem to build the wood house over the hole, so as to cover it? It does not appear to accumulate water inside (at least not for long), and the vegetation at the bottom is suggestive of a well drained soil. (I can only tell so far, because I haven't visited the place so many times yet).

And if perhaps I can take advantage of the hole-house interaction, I would love to hear about any tips.

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    Is using the existing hole as a "headstart" on the foundation excavation an option? – ThreePhaseEel Dec 15 '20 at 0:39
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    Can you? Most likely. Would there be any problems? Only someone onsite could really tell you that. As @ThreePhaseEel said, you could use it as the start of your foundation, or even a basement, but you'll need someone there to do the proper soil/drainage testing, etc. It'll be hard for us to know from this side of the monitor based on "it doesn't appear to accumulate water" and "appears to be well drained soil". Honestly, would you trust 10s of 1000s of currency (and maybe your life) on a best guess from the internet? – FreeMan Dec 15 '20 at 0:51
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    A free basement! Great. – Peter - Reinstate Monica Dec 15 '20 at 8:20
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    Nobody on the internet can tell what's in your dirt. The thing to do is to go get a soil test (aka : geotechnical investigation). Anything else is a diceroll. Engineers can tell you whether the soil will support your proposed structure or not. There is nothing to be gained by guessing other than saving a few dollars. – J... Dec 15 '20 at 13:39
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    Isn't this the definition of a modern condo tower? We dig a hole larger than the building, enough for three or four levels of underground parking, and proceed from there ... – Kaz Dec 15 '20 at 18:08
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You asked:

Would there be any problem to build the wood house over the hole, so as to cover it?

So I am going to make the following assumptions:

  • You would like to personally build the house
  • There is no plan for a basement nor slab
  • You basically plan to build the house on stilts

Here are considerations:

  • If you don't want your house to be crooked after 1 year then you need footers. I strongly advise consulting a professional as they will assess how many, how deep, and how big the footers need to be.
  • Having 1.5 meters of open space below your home is an invitation for animals.
  • Keep wood (ledger board, rim joist, floor joist, siding) several inches above the grade of the surrounding land to avoid rot problems. This is also advantageous for accessing your utilities in the future.
  • If you live in a cold region with freezing winters then your floors will be quite cold unless you use foam between your joists.
  • If you live in a cold region then a basement could be required so that your utilities do not freeze.
  • If you want to build a house on a slab then you should fill it in with dirt now and compact it as much as you can. Leave it there for a few years to let it fully compact or else your slab will start cracking on top of the uncompacted soil.
  • If you are getting a basement with foundation then presumably you will need a contractor and they will assess everything that needs to be done to the existing land. If you get to this point then the existing hole becomes a moot point as the contractor will have the equipment to do things right.
  • If you are getting a basement then I would make it sump-pump ready. If you get pooling water in the hole then install a pump; if not then a hole in the corner of your basement isn't the worst thing in the world.
  • Presumably if it rains hard enough then some water will pool in that existing dirt hole so your floor joists will experience excessive humidity moisture which is never good. Since your house will block the sun then it will take longer to dry and you could end up with mustiness.
  • If the hole is not advantageous such as crawl space storage or building a basement then I would honestly rent an excavator and fill it in with dirt from your slopes so that you create additional flat land.
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    These were super insightful tips from someone who hasn't been at the site! I surely will consider all of them to adapt or give up from the plan of building over the hole. Thanks! – heltonbiker Dec 16 '20 at 12:29
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    @heltonbiker I'm glad you like my answer! Curiously, in what region is this land parcel? – MonkeyZeus Dec 16 '20 at 12:45
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    It's southern Brazil (-29.6541606,-50.4666712), along a forest-covered hill slope. The winter is rainy, the summer is quite hot with frequent late-afternoon showers, the soil is mostly red clay and basalt. Thanks for asking, and if you have further remarks, I'll be glad to provide info. – heltonbiker Dec 16 '20 at 20:32
  • @heltonbiker Looks like a nice spot to build a home. I think you can ignore my remarks about freezing weather in the winter. I wish you the best! – MonkeyZeus Dec 17 '20 at 11:53
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Look at it as a start on a foundation hole - but don't get hung up on "making the house the same size/shape as the existing hole" because you can alter the size/shape of the hole when building the foundation, on which you will put your house, of whatever size & shape matches your ideas and budget.

You might feel more of an advantage to sticking with the hole size/shape if it was, say, quarried in solid rock and digging would be difficult/expensive, but since you mention the hole being dug, I assume lacking further info that it's in something approximating normal dirt, and normal dirt can be moved by the appropriate machines to build an appropriate foundation to set your house on.

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You don't say where you live but you will get things living down there. Rats, foxes, badgers for example. They will leave faeces and dead stuff.

Make sure you have access steps and built-in lighting.

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    I once had a housemate open up the access door to my crawl space and not close it. As it was on the side of the house away from the driveway and most foot traffic, I didn't realize it until the room over it started to smell like cat pee. And then I had the question of "do I re-secure the door and potentially trap a cat inside?" – Joe Dec 16 '20 at 19:35

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