How hard is it to put a basement under an existing house?

Author: Maureen Kessler Jr.  |  Last update: Thursday, November 27, 2025

The difficulty of adding a basement to an existing house can vary depending on the house's location, the size of the basement, and the type and condition of the existing foundation. Adding a basement is a job best handled by the pros, so you'll need to enlist professional help for this project.

Is it possible to build a basement under an existing home?

Yes, it is possible to build a basement under a house that has already been built, but it's a complex and expensive process. It often involves excavation, structural reinforcement, and potentially temporary support of the existing house. Engineering expertise and proper permits are essential.

How much does it cost to put a basement under an existing house?

Digging out a basement costs homeowners an average of $60,000–$150,000. See which factors will impact your total cost. Mark Howey is a licensed California building contractor with more than four decades of experience in the construction industry.

How much does it cost to convert a crawlspace to a basement?

Transforming a crawl space into a fully functional basement typically ranges from $20,000 for a simple dig-out to upwards of $90,000 for a comprehensive job including excavation, underpinning, and interior finishing. Costs vary depending on location, size, and the specific requirements of the project.

Can you pour a foundation under an existing house?

Yes. It's quite common to do so. It requires distribution of the weight present on the existing foundation via piers which holds the structure in place and then repairing the foundation. A structural engineer is advisable for this purpose.

How a Basement Digout is Done by Hand

How much does it cost to replace the foundation under an existing house?

The average cost to lift a house and replace the foundation is $20,000 to $100,000, depending on the home's size, accessibility, and support required. Without house lifting, replacing a foundation costs $6 to $50 per square foot or $12,000 to $100,000, depending on the type.

What do you put between foundation and house?

For small gaps, you can use caulk or expanding foam. Apply the material to the gap and allow it to dry for the amount of time specified by the manufacturer. This method is ideal for gaps that are less than ½ inch wide. You may need to use a concrete patching compound for larger or more significant gaps.

Is it worth digging out a basement?

Digging out a basement increases a home's square footage and resale value, especially if the basement is walkout or rental space. However, the project is expensive and may not provide enough return on investment (ROI) to be worth it.

How hard is it to turn a crawl space into a basement?

Expanding your crawl space into a full basement is a big job – and it absolutely has to be done right. You'll need to have the area beneath the home excavated, as well as underpinned. When done wrong, it can cause major structural issues. If you really want to do it – make sure you hire a pro and expect a big bill.

Is it cheaper to build a basement or crawl space?

Crawl spaces, however, are cheaper to build than basements (saving you between $5,000–10,000), though they take approximately the same amount of time to construct.

How long does it take to build a basement under a house?

The process, however, is long, costly, and arduous. Adding a basement to an already existing house can take anywhere from 9 to 14 months before the project is completed.

Do basements add value to a home?

A finished basement can increase your home's market value and appraisal value so you can sell your house for more. So, while square footage may not always count, a finished basement increases home value regardless.

Can you add a basement to any house plan?

Yes, any house plan can have a basement if the local zoning and landscape permit it. To map out what your basement could be in your new home, our collection of home plans with a basement includes various types of this foundation.

Is it legal to build your own basement?

In simple terms, homeowners across the Golden State can construct basements as long as they are built in accordance with the safety guidelines enshrined in the state's city codes and construction laws. Historically, basements haven't been as common in California compared to other parts of the country.

Is it cheaper to build up or out?

In general, building up tends to be less costly than building out. When you factor in things like digging and pouring a new foundation, building out typically carries a higher price tag (though this isn't always true, and certainly depends on the size of the room and its purpose).

How much does a 2000 square foot foundation cost?

A foundation costs $6 to $50 per square foot, depending on the type. A standard 2,000-square-foot concrete slab foundation costs $12,000 to $28,000, while a basement foundation costs $70,000 to $100,000+. The best foundation for your home depends on the soil conditions, water level, and frost line in your area.

Do I need a permit to dig out my basement?

Finishing a basement can be an exciting home improvement project, but it usually requires getting the proper permits first. You might be tempted to skip this step, thinking it will save time and money, but finishing a basement without a permit can lead to serious consequences that may end up costing you more.

Can you turn a crawl space into a basement cost?

Turning a crawl space into a basement is a huge project. Converting a crawl space into a basement can increase your home's living space. The project costs at least $60,000 and possibly over $200,000. Numerous professional services are necessary to complete the job.

What are the disadvantages of a crawl space foundation?

What if you bought a house built on a crawl space foundation?
  • Mold/mildew.
  • Wood rot and structural damage.
  • Floor failure.
  • Decreased insulation R-values.
  • Increased heating and cooling bills.
  • Odors.
  • Insects.
  • Dust mites.

How much does it cost to dig and build a basement?

You might need a basement if you're dreaming of extra storage space, more rooms, or a place to entertain. Digging a basement beneath your home costs $37,000 on average, although it can go as low as $12,000 or as high as $150,000. Size also impacts the cost, and you can expect to pay $75 to $150 per cubic yard.

Is it unhealthy to live in a basement?

Hazards commonly found in basements include toxic chemicals, pests, mold, indoor air quality issues (such as dangerous gases) and fire hazards that can affect all rooms of the home.

How to turn a crawl space into a basement?

This process often involves heavy digging machinery and hundreds of labor hours spent digging the crawl space and transporting the dirt out. Once that's done, a professional concrete contractor must extend the foundation down to the new level and ensure that it will support the weight of the home above.

Should I fill a gap between foundation and soil?

We are getting a lot of phone calls regarding this space between the edge of your foundation and the DIRT. 🛑 Do Not fill it with Dirt 🚨 Do Not stick a hose down in the crack. Soaker hoses or a drip line are the only safe way to gradually bring the soil back to support the foundation.

What is the best base for a house foundation?

One of the best foundations for a house is slab-on-grade, plus the addition of a basement. Concrete slabs are cheap and easy to install, and the cost of materials is inexpensive. Adding basements expands usable square footage and increases the home value.

Should I caulk between house and concrete?

Because expansion joints are open gaps leading straight to the soil and sub-material under the concrete, they should always be caulked to prevent water from eroding away the base that the concrete is sitting on.

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