Can You Build a Basement Under an Existing House?

Due to rising real estate costs and the need for more living space in urban areas, building basements beneath existing homes has gained popularity. The tendency was first restricted to upscale neighborhoods.

However, as property values have increased, “dig-out” basements are now economically viable in the majority of neighborhoods as well as other affluent regions.

You can build a basement under an existing house as long as you comply with the construction laws of your state. You get more home space without changing the compound by much. It will cost you about $110,000 for 2,000 square feet of basement area without an existing crawl space.

Underpinning is a process used to make habitable space beneath an existing structure. To safely remove dirt beneath the building to create the new basement area entails raising the depth of the existing foundations.

Underpinning depends on a wall’s capacity to span unsupported for a brief distance. This enables a portion of the wall to be dismantled so a new building can be erected underneath it. So long as the existing wall is supported for its entire length, a new lower-level basement foundation, and wall portions can be constructed.

Basement Under Construction

How do you build a basement under an existing house?

Although adding a basement beneath an existing house is possible, you should consider if the effort and cost are worthwhile. The cost of renovations can increase your home’s worth. As you choose how much to invest in the project, consider the size and breadth of your new basement project.

The main steps when constructing a basement are as follows:

  1. First, you start by building a solid foundation for the robust support system underneath the home that your new basement needs. 
  2. The next step is constructing a new basement, a time-consuming process that calls for caution and knowledge. In certain instances, digging is carried out by hand, which may be prohibitively expensive.
  3. You then make the walls. The walls and foundation will be built once the concrete columns supporting the home have been removed to create a place for the new basement.
  4. Thermoregulation and waterproofing will be your next focus as the new subterranean area needs defense. It is necessary to waterproof the base, the walls, and the floor and exterior of the walls. Drainage and humidity will be crucial factors to consider for the newly finished basement. Your basement needs sufficient drainage, whether it has a kitchen, bathroom, or utility room.
  5. All that remains is for your builder to finish the basement area to your liking after all these stages have been accomplished. 

Then, finally, the room will be perfect for your house with flooring, cabinets, trim, and other additions you have made to your preference.

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

Adding a basement to an existing house costs between $20,000 and $150,000. Unfortunately, there isn’t a precise number because any basement conversion project’s budget is unique.

Whether there is an existing subterranean space underneath the house is frequently the most crucial factor. If there is, expanding the already completed work can significantly reduce costs. A new basement will cost more because more work will be required.

Other crucial factors include:

  • The basement conversion’s size.
  • The difficulty of converting the basement.
  • The components of construction.
  • The furniture, décor, and interior design.

You should be aware that creating a basement is a significant undertaking that needs a sizable budget to be finished securely. Therefore, always employ specialized service and the best tools for the finest outcomes.

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

Like cost estimates, predicting the precise amount of time needed to construct a basement is challenging. This also relies on several factors, but you can anticipate that any basement conversion will take about 6 months to complete.

It’s crucial to remember that several key steps are involved in the design, excavation, and construction of a basement beneath an existing property to help you visualize the process and how much time this may take. 

These are:

  • Preliminary design, planning, and surveying
  • Construction and excavation
  • Interior decoration and design

Naturally, these variables will differ. For example, a basement conversion will take longer to complete the larger and more complicated it is. It can take less time if you already have a cellar, but digging a basement from scratch will take much longer.

Can you add a basement to a slab house?

It is possible to add a basement to a slab house. The method your builders will use to construct your basement will be similar to that used to build homes with crawl spaces.

Workers must break the concrete slab foundation to delve beneath it because a slab house depends on it. Then, a new foundation and basement will be installed, and all utilities will be restored. 

Finally, the house will be lifted and supported; the foundation will be removed, dug up, and your home will be returned to its original location.

Although it is conceivable, the additional work your contractors would have to perform could make it quite pricey.

Where can’t you build a basement?

Some of the areas you can’t build a basement in include:

  1. Areas with underground soil containing high moisture levels make basement construction infeasible. Any attempt to construct a basement in such a location exposes the basement to flooding. Although using a relief well is a possibility, there are more affordable choices. Another risk is that the well’s pump can malfunction.
  2. Areas with soils that have a lot of clay in them. Based on the weather, this soil tends to expand and compress significantly. It expands when wet and contracts when dry. They are not suited for foundation building due to their shrink-swell cycles, which cause foundational movement that is harmful to any home.
  3. Areas with softer strata like limestone bedrock and shallow soil layers. Such a large amount of rock requires extensive excavation. You’ll need to hire an engineer and purchase powerful machinery. Before building a basement in the South, you must obtain a price quote for the excavation charges.

Basement vs. story floor, which is better?

You can elevate your brand-new home by adding on the floor. However, a further cost-effective option is to build below ground level if adding another story differs from what you had in mind.

Building up (story) is always the most affordable approach to increasing the square footage of your home because it takes less labor and materials. In addition, you don’t have to give up any of your yards when you build up, which is another benefit.

However, the best way to add a low-cost extra bedroom, family room, or exercise area is by finishing or adding a basement. It provides a safe location for you and your family during severe weather. Giving homeowners peace of mind. 

Despite all these advantages, most localities set a maximum height for houses when it comes to building up. Stairs must also be considered since they may dramatically alter the floor plan. They can easily take up 80–120 square feet or more of interior space; this addition may reduce your available room. 

Building outward enlarges the footprint of the house and necessitates greater yard excavation. In addition, moving around sewer and electrical lines could increase your bills. 

In conclusion, building up (story) is ideal for individuals trying to conserve money, while building down (basement) is the best for people who reside in weather-prone areas.

Benefits of adding a basement to a house

If building a basement is on your to-do list, you probably have a storage or space issue. If there is space below, why not construct there instead of continuously building up? This is especially true if your home has a crawl area underneath it.

Some of the benefits of building a basement include the following:

Additional storageMay be inaccessible
Living or entertainment spaceHigh building cost
Increase in home valueDraws in pests

1. Additional Storage

A new basement can significantly increase your storage capacity if your bedroom closets contain extra baby gear, clothing, or seasonal decorations.

2. Living or Entertaining Space

Basements also give guests a place to hang out while you’re cooking dinner, away from the kitchen. A bonus: The concrete walls are an ideal overflow room for huge gatherings and provide adequate soundproofing for your children’s drum set.

3. Increase in Home Value

You’re likely to experience a boost in the value of your property because you’re adding square footage. However, you must waterproof your basement to ensure that it keeps its value.

The downsides of adding a basement to a house

While basements have many advantages, they also have some disadvantages. Flooding, upfront expenditures, and whether it’s even feasible to create one in your home are all things to consider.

Some of the downsides to a basement include the following:

1. Inaccessible in some places

Basements are quite uncommon in states like Florida and Georgia. This is because it is impossible to construct more than a couple of feet into the ground due to the climate and local aquifers. In addition, these places’ unstable soil makes it likely that the basement may collapse.

2. High Building Cost

A basement addition to an existing home does raise its value, but it is unlikely to make up for the construction costs.

3. Draws in Pests

A basement is a perfect place for bugs, even though the spooky monster tales you heard as a child about the cellar were untrue. Basements are a prime choice for all the creepy crawlies and vermin if not correctly sealed due to the high moisture content and dim lighting.


In conclusion, if you want to make the most of the space in your current house and don’t have any immediate plans to move, consider adding a basement. You will receive the maximum amount of square footage while avoiding the hardship of relocating to a larger house.

A basement might not be an option if you reside in a warm climate. Building a room or finishing the attic is a better alternative if you merely need an extra bedroom, as acquiring a basement requires a significant investment.

Frequently Asked Questions

How do you put a basement in an existing house?

Adding a new basement to an existing house involves moving the house to an adjacent plot of land, excavating the basement, then building a new foundation for the house before returning the house to its place.

Can you dig a basement next to an existing house?

You can build a basement under an existing house or next to it. You can link the new basement to the main house through a staircase.

Can you turn a crawl space into a basement?

You can turn a crawl space into a basement with the right planning.

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