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So first off, I love basements, its almost like a fetish. Ook.. so this guy is selling a basement in his building and its perfect(got good water, electricity, comms, location and cheap!) except for the fact that the ceiling is 2m from the floor, since I'm 2m tall that is a problem.

I asked him whether I could smash the floor and put a new one maybe 10cm lower, he said I can do whatever I want for all he cared and as far as he knew, there was nothing important underneath.

Ok so now I'm thinking big! Maybe go ahead and lower it by 40cm? or 300cm? Dig another floor underneath the building!

The floor is 40m2 and the concrete is ~8cm thick according to him. The brick walls should go down another 20cm underground. How realistic would it be for me to do that? Could I leave some space between the walls and the new floor shape to dig extra low? Obviously I'd get some help and I know a guy who knows a guy who has a jackhammer.

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    I would not buy ownership in a building where the majority owner tells people 'sure do whatever you want' to things like foundations. :)
    – DA01
    Commented Aug 25, 2014 at 16:24
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    Lowering the floor will usually require underpinning the foundation footings. This is not a DIY task and requires proper engineering.
    – Steven
    Commented Aug 25, 2014 at 16:26
  • People dig out basements all the time to raise ceiling height. This is done a ton of the east coast in high end neighborhoods. Usually around 30K for medium sized basement. I don't understand how the hell you could do this just owning the basement. Obviously there is risk involved with the upper levels.
    – DMoore
    Commented Aug 25, 2014 at 17:27
  • Afaik, the majority owner is some swedish dude. This guy just owns the basement, there are 2 more families in the house. Considering what the basement is worth and how much it would cost to lower the floor I'm probably not going for it after all :/ All though, wouldn't it be hilarious if I just moved in and started digging and smuggling out ground while everybody else is away? xD
    – Jake Freelander
    Commented Aug 25, 2014 at 17:55
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    Law of Unexpected Consquences: news.nationalpost.com/2012/06/29/…
    – DJohnM
    Commented Aug 25, 2014 at 20:20

4 Answers 4

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The concerns I'd have -- outside of whether there's anything under the floor that could be disturbed, and whether there's a rock layer under the floor that would prevent your lowering it -- would be:

  1. You're going to have to make sure this doesn't disturb the foundation. I'd suggest getting an engineer's advice before doing anything.

  2. You're going to have to dispose of all that rubble. That means renting a dumpster, and hauling it out of the basement. Not impossible -- a friend of a friend put a basement under an old farmhouse by digging outward from the root cellar, putting the house up on jacks as he went, and then getting a foundation installed under the house, and he did almost all of the digging and hauling himself -- but it's a nontrivial amount of work. You're going to be moving several tons at least.

  3. Putting in the new floor is going to require establishing a good seal against water infiltration between it and the foundation walls. Again, I'd suggest getting an engineer's advice.

So: Yes, theoretically it can be done. Personally, given the cost and effort and potential liability involved I wouldn't consider doing it on property I didn't own. And I definitely wouldn't consider it without expert advice.

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Consider this simplified diagram of the bottom of a house.

enter image description here

The brick wall is part of the house; It has to carry the vertical and horizontal loads of the structure. Two story or eight stories, will look very different. But the design of the house will be independent of the soil conditions at the site.

The green strip represents the footings. (They rest on top of the brown surface) They are the interface between the structure and the soil. Their job is to accept the load of the structure without breaking, and spread that load out over enough area so as to not overstress the natural soil (the brown part). It's the job of a soil engineer to decide on the thickness and width of the footings, based on the house design and the local soil conditions.

Now look at what you may wind up doing:

enter image description here

In particular, consider the red vertical surface. This is just natural soil, the same as before, but now without and lateral support. You've just incorporated a pile of dirt into the foundation of your house.

Short answer: hire an engineer.

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Given a willingness and the ability to commit all the necessary resources, lowering the floor is entirely reasonable. Whether it makes economic sense or is even economically viable depends on the local construction and real-estate markets, the regulatory environment, and the financial, equipment and labor resources at the Owner's disposal.

Viewed as a real-estate development project, the general rule for less experienced developers without a track record is to be more prudent in regard to problematic sites than a more experienced real-estate developer might be imagined to be. One never knows what is under the basement of an old building and one should expect to be pleasantly surprised whenever it is not a can of worms.

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The much more common solution for that problem is not to lower the basement floor, but to raise the house.

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  • Here's a recent news article from my area quoting a price of $150,000 for a man to raise his house 11.5 feet.
    – mbeckish
    Commented Aug 25, 2014 at 15:36

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