When buying a house one of the scariest things you can hear is that it has a foundation issue.
How big of a deal is a foundation crack?
My recommendation is always to get a foundation contractor for further evaluation. But let's discuss what options they will present you with.
For starters let's discuss what a foundation is. The purpose of a foundation is to transfer the weight of a structure to the ground below it. If we would build a house directly on the dirt, the house will settle, shift, and eventually fall down. A solid foundation is designed to transfer the weight of your house to the dirt under it.
When the foundation contractor comes, he will need to determine the cause of the cracks. Most cracks are caused by soil movement. Soil movement is caused by moisture getting into the soil eroding it away, expansive soils that expand and contract depending on the season, or earthquakes. When the soil moves, the concrete has nothing supporting it, and the concrete cracks. The 1st thing you need to do in these cases is prevent moisture from getting there. In most cases installing gutters, and removing planter beds will solve it, and in other cases, you need to have a grading contractor regrade the area, and install drainage.
When you have expansive soils it's fairly typical to see seasonal cracking in the corner of window and door frames on the drywall and stucco. If you try repairing these cracks, they will appear next season when the soil moves again. If the cracks are smaller, and only affect the cosmetics, it may not be worth doing anything about. (But of course, you would need a foundation contractor to ensure it's only limited to cosmetics)
Once we determine the issue and you make a plan to stop future damage we need to repair the cracks. If the cracks are extremely small hairline cracks, very often preventative maintenance is enough. If they are medium (usually big enough to fit a coin) they can easily be repaired. The 1st step is to fill it with epoxy to prevent moisture from entering and rusting the rebar. Once it's filled with epoxy we need to make sure the crack does not spread. This is done by bolting metal straps in the crawl space. A repair like this will usually run around $1,000 a crack. (As of 2022)
9 out of 10 times that will be enough, although in worse cases, underpinning may be required. Underpinning is done when the dirt directly under the concrete is not strong enough. Underpinning refers to digging under your foundation and adding concrete to give more surface area in the dirt. Usually, the new and old concrete will be joined together by dowling rebar into the old foundation that extends into the underpinning concrete. These repairs can get into the thousands.
It's very rare to see, but very few foundations will need partial or full foundation replacements. This is usually only required when there are portions of the concrete, with large horizontal cracks, that are settling at different rates, or concrete that is extremely deteriorated. Concrete usually has a lifespan of 100 years, so it's more likely in older houses. These repairs usually require a structural engineer, and can easily get into the tens or hundreds of thousands.
Hillside homes are far more complex due to the gravity always wanting to pull the house and dirt down the hill. When buying a hillside home, it's always a good idea to get a geologist to give you a geological survey so you can understand the condition of your land.
In California, one of the largest concerns is earthquakes. During the 1994 earthquake, houses were falling off of their foundations. Most houses built in the 40s and on were bolted to their foundation, but these bolts were not strong enough and did not hold up on houses near the 1994 earthquake epicenter.
Due to this, standards were made for new construction on how to properly bolt down a house. Most foundation contractors will always be trained to look at the bigger picture and will also include a quote to "retrofit" your foundation, bringing it up to modern earthquake standards. You can usually get a large discount if you repair your cracks, and retrofit your house at the same time.
In summary, foundation cracks can range from just needing to add gutters, to hundreds of thousands of dollars worth of work. Getting a foundation contractor to give a quote is the best plan of action, but in most cases it can easily be repaired.