Another basment under my current basement

[nathan_m_2005]

Right now I have a cement basement in a split-foyer house. I have a big problem with storage in this house, and there isn't really much room to add on around the house. I was wondering if it would be possible to dig down underneath the current basement and make another basement below that for storage. Think its possible?

[JWmHarmon]

One method for adding a second basement level is to jack the house up, slide steel beams under it, extending them over solid ground outside the house, then demolishing the current foundation and floor. Then you would dig down enough to build a two-level basement. This method is extremely expensive, but it can be done. You would be much farther ahead to rent a storage space for your extra stuff at the nearest storage building. You might even consider buying a different house.

Another solution is to build a separate room outside your current foundation. This needs to be a reinforced retaining wall design. This storage space could be as little as four feet wide. You can cover this with a deck with rubber membrane waterproofing with pavers or decking over the waterproof membrane. Four feet of space does not extend far beyond the house, but gives a great deal of storage area. You can line the walls with shelves and still have enough space to get past the full length of this storage area. This also is quite expensive. You can build as much wall as you want, completely surrounding your house, or limiting it to one wall, or one section of one wall.

I did this for a client some years ago. We cut a door from the basement into the storage space. It was 4 feet wide and built on three sides surrounding one end of the house. The total length was one hundred feet long. Shelving units are two feet deep. I braced the retaining wall against the foundation with flying buttresses. Extra care was taken to tuck the rubber membrane under the bottom piece of siding. I also installed a hatch with a ladder to the outside. It has performed satisfactorily for the last 15 years.

This particular project gave the owner the potential for 500 linear feet of shelving, spaced at 16 inches between shelves, minus 3 feet for the access door. Since the sewer and water line were uncovered during the construction process we installed drains for a future half bathroom in part of the space.