Building a basement next to existing foundation
Actor 123
the house is a bit on the small side so I am a planning on adding
approximately 1100 sq. ft. into the back yard (which would bring me to
the maximum allowable square footage limits in the city). While I am
at it, I am considering building a basement under the new addition to
gain additional square footage (basement space doesn't count against
the city limits).
My question involves how close I can come to the existing foundation
with this new basement? I've searched the web but all I can find is
info on building a completely new house, or adding basement space to
an existing house. This is sort of a hybrid. I want to add brand new
basement space right next to an existing house, but NOT dig out under
the existing house at all (only under the addition).
Can I build the new basement wall right up against the existing
foundation, or do I need to leave some space? If so, how much? The
house is in California, if that matters. Thanks
Bruce
You really should contact a soils engineer to answer your questions
for you. The type of soil, soil compaction, design of the existing
footings, etc. will all come into play.
Try this ... Draw a line 45° out from the bottom of your footing; you
should not remove any soil from below this line. (In other words,
calculate the depth of your excavation below the existing footing and
do not excavate any closer to the edge of your existing footing than
that.)
Now if you want to install shoring then you will be able to excavate
right up to the footings. But that will cost extra.
Tom Baker
Any seismic activity will need to be addressed in the way the two
structures are connected.
The new and old buildings may move in different ways and cause stress.
TB
Bill
existing foundation, then fill them with rebar and concrete. Maybe first
holes would be every 5 feet, then subsequent holes would be between those.
Basically you are building a retaining wall a little at a time.
Then you can excavate your basement right up next to the concrete filled
holes.
I think I have seen this done when they are building a high-rise (with a
parking garage underground) next to an existing building or street.
"Actor 123" wrote in message
Actor 123
Thanks for all the quick and helpful responses. What about just
replacing the foundation wall with the new basement wall? The
basement only touches the house on one side. Would this be easier or
harder than the suggestions already mentioned?
Bill
>
> basement wall? The basement only touches the house on one side.
>
That is possible. Easier if it is a wood frame house. They could place a
large steal beam under that side of the house to hold it up during
excavation. House movers do this type of thing before moving a house.
If going that route and it is a wood frame house, you could even extend the
basement under your existing house as well. Moving air conditioning, water
heater, laundry, etc. to basement could also make more room upstairs.
May want to check on costs for the different plans. Ask around for
architects who have experience with this type of thing.
Js Walker Lazenby Jr
than virtually any other method. You haven't said whether
your addition will be like an 'L' leg, like the foot of a
'T', or completely from side to side of the original
footprint. Nor do you say how deep your existing foundation
is.
Either footprint will require that you step your footing
down from the original elevation to the elevation of the
basement footing. One easy way to do this would be step
down in incremental steps (thus, a stepped footing . . .
clever, huh?) from the line of the original footing to your
new footing depth with a graded crawl space between the
original house footprint and where you have reached basement
depth.
That really doesn't make economic sense, however, so you
need an engineer type to design a transition from the
original footing to the new. He/she might create a virtual
retaining wall between the old and the new or some other
scheme; however, you can't just simply add on a deep,
basement footing adjacent to a shallow, crawl-space footing.
A competent foundation contractor or a structural engineer
should be able to advise you in advance of your incurring a
significant design expense. I'd be calling one or the other
right about now. Any hard and firm advice you get on a
newsgroup (and this is the best one there is) is going to be
unreliable, as no one has the necessary information to make
a sound decision or give sound, reliable advice.
It IS doable. It is NOT cost prohibitive. It IS likely to
cost you more than a partial second story addition, however.
Jim
Actor 123" <act...@aol.com> wrote in message
news:6c1d2b63.04050...@posting.google.com...
Actor 123
> Not only easier but far more likely to create fewer problems
> than virtually any other method. You haven't said whether
> your addition will be like an 'L' leg, like the foot of a
> 'T', or completely from side to side of the original
> footprint. Nor do you say how deep your existing foundation
> is.
Its going to run completely adjacent to the existing house on one full
side (there are some juts in and out on the existing house, but the
new addition would be adjacent to the old house for the entire run of
the side facing the back yard)
I'm not sure about the depth of the existing foundation. There is a
crawlspace underneath the house, rough guess 2' between the house and
the dirt, but I'm not sure how deep the footers run. I've bought the
place, but haven't taken possession yet, so I'm a little shorthanded
when it comes to facts...
>
> Either footprint will require that you step your footing
> down from the original elevation to the elevation of the
> basement footing. One easy way to do this would be step
> down in incremental steps (thus, a stepped footing . . .
> clever, huh?) from the line of the original footing to your
> new footing depth with a graded crawl space between the
> original house footprint and where you have reached basement
> depth.
>
> That really doesn't make economic sense, however, so you
> need an engineer type to design a transition from the
> original footing to the new. He/she might create a virtual
> retaining wall between the old and the new or some other
> scheme; however, you can't just simply add on a deep,
> basement footing adjacent to a shallow, crawl-space footing.
What about Bill's suggestion about just replacing the old foundation
on that side with a new one, temporarily keeping the house up using a
steel beam?
>
> A competent foundation contractor or a structural engineer
> should be able to advise you in advance of your incurring a
> significant design expense. I'd be calling one or the other
> right about now. Any hard and firm advice you get on a
> newsgroup (and this is the best one there is) is going to be
> unreliable, as no one has the necessary information to make
> a sound decision or give sound, reliable advice.
>
No worries, I use newsgroups for ideas I get about the design, but I'd
never even think of going forward without a professional. I've found
newsgroups extremely helpful for this type of freeflow brainstorming,
though.
> It IS doable. It is NOT cost prohibitive. It IS likely to
> cost you more than a partial second story addition, however.
>
Unfortunately, that is not possible. The planning commission in my
area has pretty severe square footage limits, and the overall square
footage allowed for a two-story house is actually less than that
allowed for a single story. Basements, however, do not count against
the square footage limits. I'm trying to maximize the square footage
of the house.
Dan Listermann
existing stone foundation basement. The trick is that the garage's floor
will be 30" below the basement's floor. We are going to step out with a
retaining wall.
"Actor 123" <act...@aol.com> wrote in message
news:6c1d2b63.04043...@posting.google.com...
Js Walker Lazenby Jr
in message news:<zPSdnePI5fQ...@gbronline.com>...
> >
> > Either footprint will require that you step your footing
> > down from the original elevation to the elevation of the
> > basement footing however, you can't just simply add on
a deep,
> > basement footing adjacent to a shallow, crawl-space
footing.
>
>
> What about Bill's suggestion about just replacing the old
foundation
> on that side with a new one, temporarily keeping the house
up using a
> steel beam?
The problem with that suggestion is it does not provide for
stepping the footing down, incrementally, from the existing
footing level to the addition's footing level, which is also
going to be the new footing level for the existing wall
abutting the addition.
Let's say for descriptive purposes that your new
"party-wall" between existing house and addition runs
north/south. That is the foundation wall that you will be
extending down to the proper depth for your new basement.
(Whether or not that wall is a continuous, straight line or
one that juts in and out.) If your basement is going to
extend all the way along that "party-wall," you will be
extending the foundation wall all the way along that
"party-wall." You will need to excavate east/west a
sufficient distance under the existing east/west walls
abutting your addition to allow you to step UP your new
footing, incrementally, under the existing foundation, to
reach the level of the existing footing. The last, most
shallow, new footing will likely extend just a bit further
under the existing footing, with the new foundation wall
extending up completely abutting the underside of the
existing footing above.
Some splinter beams might be necessary to support the
existing building during this. I say "might." If you do
not disturb (excessively) the existing footing either on the
major north/south run or on either abutting east/west runs,
but coordinate excavation with closing up the new foundation
walls, you likely can eliminate any need for splinter beams
or otherwise. (I'd make some provisions anticipating
cave-in, etc., but you'd be surprised at what you can take
out, bit by bit, and build under.)
It is difficult to verbally describe this, particularly when
avoiding trade argot.
I say don't desturb the existing foundation wall or
foundation footing, but after you have the major,
north/south footing and foundation wall in place, you can
knock off any of the old footing that encroaches in your new
basement. You're going to want to cover that wall, though,
as it is not going to be a pretty sight on the inside.
Jim
lauravan...@gmail.com
Thanks!