Why Porches Can Be a Problem
If your house has a front porch it is possible that the foundation is under the front of the porch as shown by the red arrow rather than under the floor below the front wall of the house, which is where the house floor is as shown by the blue arrows.
Go stand on your front porch and look down at the floor in front of your front door. Notice how it is a few inches below the house floor and you need to step up 2 or 3 inches up from the porch floor to get into the house. This is because you have two floors independent of each other. The porch has a floor and the main house has a floor. Oftentimes there is foundation under the front of the porch, but no foundation under the front of the house floor. Where the porch joins the house we will find posts supporting both the house floor and porch floor and no foundation. Post readily tip over in earthquakes and this is a serious weakness.
It is an engineering principle that shear walls must be built on all four side of a house as is required by FEMA P-1100, Appendix Chapter A3, Standard Plan A, and all the other seismic retrofit guidelines. Many contractors do not understand that building a shear wall under the front of the porch does nothing to protect the house. We have seen contractors do this many times.
To the left you see a retrofit where a contractor did not understand the porch and the main house have two separate floors. The green lines represent shear walls. The contractor put shear walls on all 4 sides of the house, but did not understand the front of the porch is different from the front of the house. The house is as susceptible to earthquake damage as it was before the retrofit. There are only posts support the front of the house floor and the back of the porch floor and these readily topple over in earthquakes.
In these cases there is no other solution other than add foundation under the front of the house and attach the floor to a new shear wall. It is easy to put in a new foundation on either side of the new foundation so we do this as a matter of cours.
In the illustration below you can see how this looks from inside the crawl space. You can see the two separate floors, the posts that support both the front of the house as well the back of the floor, and the existing foundation at the front of the porch. It is not necessary to put a new foundation under the entire porch, just a segment long enough to resist anticipated earthquake forces. If your house does need foundation work make sure all the regulations on foundation construction are followed. The red arrow illustrates where minimal earthquake forces will cause the posts to tip over.
Here is an example where the foundation followed the front of the porch. For some reason this is almost exclusively found in homes that have porches that span the full width of the house.