Make your own basement

June 4, 2009 8:50 AM

I recently bought a house that's an duplex joined on both sides. The basement is a ~2 foot crawlspace (though one small area is ~4 feet). I would like to excavate it into a basement. How could this technically be accomplished?
posted by Napierzaza to home & garden (15 comments total)

I don't know anything about anything....

But I think they would take steel I beams, run them under the house, jack up the house off the current supports slightly (from the outside, using the beams), excavate entirely around it and under it, build foundation molds, drop in a foundation, and then rest the house on the foundation.

Think $25-$50k depending on the size of the house.
posted by SirStan at 8:53 AM on June 4

Looks like I misread your question -- you dont want to fill in a full foundation -- but rather dig out whats currently there for more space...
posted by SirStan at 8:54 AM on June 4

I live in an old town-house neighborhood, and I know some folks have done the basement-excavation thing. It's a big deal, since foundations don't generally go down as far as you'll want to excavate, so there must be a lot of engineering to keep things supported during the dig. I know also that the nighbor's permission must be obtained.
posted by MrMoonPie at 9:16 AM on June 4

There was just an article about doing this in the Washington Post Real Estate section.
posted by OmieWise at 9:18 AM on June 4 [1 favorite has favorites]

2nd what MrMoonPie says. There was a recent article in the Washington Post that talks about doing it in DC. The general focus in the article is on the cost vs return of doing it, but they have some details on the process, and the risks that go along with it.

In addition to the article, I've read some things on one of the DC mailing lists that talk about one contractor that's done a very poor job with this, leading to serious problems for the house they're working on, plus the neighboring houses.
posted by inigo2 at 9:20 AM on June 4

Damn my slow proofreading...
posted by inigo2 at 9:21 AM on June 4

Funny--I hadn't seen that article, but most of the homes it talks about are in my neighborhood. I bet we're all talking about the same places.
posted by MrMoonPie at 9:27 AM on June 4

Go here. Be warned, since this is a duplex you may be breaking terribly vindictive laws, but there ya go.
posted by aramaic at 9:54 AM on June 4

When excavating for pipes, it is prohibited by code to dig deeper than the imaginary line that
starts at the base of the footings of the foundation, and proceeds downward at a 1:1 rise:run
ration. This prohibition protects the footings from having their support removed from them.

For example, if you were 2 feet from your foundation wall, and trenching for a pipe that was
to run parallel to the wall, and your footings were 2 feet below grade, then you could dig
no deeper than 4 feet in your trench.

Likewise, the steepest stope that you can support, for the short term, is 1:1 (or 1.5 to 1,
for some soils).

These prohibitions and requirements assume that you will be refilling the excavation. For
a dugout like I'm assuming you want to build, you would have to much more conservative,
or build an engineered retaining wall at the perimeter of your excavation to make sure
that you don't remove important support from your perimeter foundation. Either way, you're
going to be talking to a geotechnical engineer.

Then there is the water table. In some areas it is close enough to the surface that you
might have problems, perhaps seasonally, with excess moisture in the crawlspace from
the exposed water table. Think moisture and mold. This can be solved by sumps and
pumping, but it's something that you want to know about in advance.

Check your grant deed, and find out how the crawlspace is is treated. There is the
possibility that the utilities in the crawlspace are shared between the duplexes, and that
there are restrictions based on that. Check out the term "party wall".

I am not a geotech, lawyer, structural engineer or building inspector.
posted by the Real Dan at 10:47 AM on June 4

You may be trying to do a 'secret' evacuation (like 'the great escape') without mvoing the house and I wouldn't even THINK of it. The dirt around the house , under the house and in your crawlspace is holding it up The logistics of shoring up as you go, avoiding pipes and services and preventing flooding would be a challenge for an MIT engineering team and wouldn't be secret considering the equipment and supplies necessary to do it properly. If done wrong, the best possible outcome is that the house would shift and cause multi ten's of thousands of dollars in damage. Yes, that is the best outcome.
posted by CodeMonkey at 11:06 AM on June 4

I've done this! Kinda... I wasn't there for the whole project, but it came down to this:
We dug a little bit, about two and a half feet, and then lifted the house about two feet. This gave us enough space to start the real digging with a back hoe.
This was done entirely by the home owner and some people helping, except for pouring concrete.
Actually not that hard, but it took a loooooong time, and after buying tools, tons of steel, a back hoe and a dump truck the home owner only saved a little bit of money. If he had contracted the whole thing out it could have been done in a few weeks.
There's contractors that specialize in this.
posted by gally99 at 11:23 AM on June 4

CodeMonkey: you're pretty negative, pretty late in the thread. You say it's impossible without tons of damage after people either said they did it, posted an article on its feasibility or mentioned specific contractors who actually specialize in it.

Thanks for all the info everyone, it looks like it's a long hard road indeed.

I know that I don't share services anymore, that was changed before I bought the place. But considering I share a wall with two buildings I can't jack anything up now can I?

I didn't mention this before, but my next-door neighbour actually had this done for him. I've only owned the building for ~2 months so I haven't spoken to him. I just wanted to see what would be involved, and if I might do it myself.
posted by Napierzaza at 12:22 PM on June 4

I thought you bought the whole building.
If you can also dig in front of or behind your property then you can make room for a back hoe and you won't have to lift it.
posted by gally99 at 12:59 PM on June 4

Well it's a row of 3 duplexes and another attached building. If I could afford that I'd pay to have the world moved down and away from the building!
posted by Napierzaza at 2:01 PM on June 4

You say it's impossible without tons of damage after people either said they did it, posted an article on its feasibility or mentioned specific contractors who actually specialize in it.

No. The advice given was to refrain from doing it piecemeal and secretly. I'm not sure whether you were really proposing this, but CodeMonkey is correct that it would be almost certainly disastrous.

The problem with a double house is that I can't imagine it being done without doing both parts of the house at once. Being "joined on both sides" (presumably to other buildings) is a probable deal-breaker. I recall (but couldn't find quickly) a Chicago news article about a condo being built in place of a brownstone (that would probably have made a perfectly good condo, but Chicago had a lot of this unnecessary redevelopment in the 1990s), where a new basement excavation led to structural problems with neighbor buildings that were close by but not immediately adjacent. The liability risk is significant.

But you will never know until you get an evaluation by a structural engineer.
posted by dhartung at 10:24 PM on June 4