How To Build a Basement Below Your Older Home

Basement Older HomeOne of our clients recently asked us to help them, decide how to build a basement below their house. Normally, a basement is built before the house is built so that the basement foundations can be used to support the house. In this case, the house was built in the 1920’s, and the owner wanted to add a basement and first floor addition to the property.

This home is an excellent example of creative adaptation of an historic home that’s nearly 100-years old. Located in a beautiful older neighborhood in an historic Southern California city, this property had stood the test of time for generations and yet was simply too small for the everyday needs of the modern family living there today.

The Challenges

As you frequently see in home renovation videos or on television shows, a word of advanced warning, “Please don’t try this at home by yourself.” This was not a do-it-yourself (DIY), handy-man or your friend the contractor type of project for reasons beyond the obvious. It required soils analysis and structural studies, plans and drawings, permits, insurance, inspections, etc., which required the services of a licensed, bonded professional engineering firm.

So, how to support the house while excavating a 10-foot deep hole below the nearly 100-year old foundation? Helfrich-Associates first analyzed the excavation site. The soils below the house were stiff clay that was calculated to support a vertical excavation side to a depth of more than 10 feet. Whenever undertaking a home expansion project of any kind, it is always important to know and understand the soil composition below the surface.

The Solutions

The existing foundations for the house, however, were not structurally sound enough to support the house at the edge of a deep excavation. The deterioration of foundations under historic homes is common because of lax or non-existing construction standards, inferior materials, the effects of time, etc. The modern solid concrete slab construction technique is a relatively new innovation in home construction. We also had to consider a nearby garage and property line walls that would be affected by the excavation.

We designed a replacement foundation for the existing house that was to be built before the excavation started. This new foundation system was then underpinned to support it while the basement excavation was made within inches of the existing house foundations.

A temporary structural support also needed to be erected while the adjacent excavation was being done to ensure the slope above the property remained in place. This project is an example of the types of projects that rely on close cooperation and coordination of soils and structural engineering, along with the construction team during the excavation and build phases.


Normally adding a second story is the typical approach in these situations, but going up also can require reinforcing the foundation due to the added weight of a second story. In well-established historic neighborhoods, surrounding homeowners also often either have view properties or there are other restrictions or expectations to maintain historic district integrity.

Sometimes “going down, please” is the most practical option for homeowners. There are also green energy-saving passive cooling advantages to basements that can trim the use and cost of electricity in warm climates versus a second story option. In addition, the upgrading of the foundation for this homeowner not only supported the expansion, but also protected the existing structure from further deterioration from the effects of settling and added a modest level of earthquake resistance too.

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