Can I Add A Sub-Basement To An Existing Basement: What To Consider

A completed home could still transform from time to time. Existing basements can be upgraded by adding a sub-basement. In building and construction, additions and extensions aren't a new concept. Homeowners might be interested in adding a sub-basement to an existing one, but is it possible? Well, we asked the experts, and they gave us some answers.

The answer is yes! You can add a sub-basement to an existing one but it will be very costly. Before you undertake this addition, you must have an expert's opinion. Consider the following to avoid damaging the rest of the basement and foundation of your house:

  1. Topography
  2. Property structure
  3. Geology
  4. Waste drainage
  5. Access
  6. Fire escape
  7. Additional insulation, height, and light

Once the above considerations and requirements have been implemented, you'll have additional square feet in your home. Homeowners might not all be in a position to do the basement and additions extensions by themselves. Therefore, professional help is necessary. Continue reading as we clarify the above requirements and considerations.

basement renovation, wood cuttings on the floor, exposed wall framing. Can I Add A Sub-Basement To An Existing Basement What To Consider

What Are Sub-basements Used For?

The primary use for sub-basements is storage. This additional space could vary in size due to the cost of construction and the topography.

Sub-basements in homes can be utilized as wine cellars, playrooms, or pantries. Some homeowners might need additional bathrooms, gyms, or home theaters. Basements that have been converted to apartments or family rooms might need additional space.

In residential homes, sub-basements might be used as an area for storing boilers and furnaces.

Sub-basements are not something rare in commercial buildings. In commercial buildings, they are utilized as full-blown basements. This is because commercial building owners can afford the cost of constructing a fully functioning sub-basement.

In commercial buildings, sub-basements house swimming pools in hotels, vaults in banks, and museums or offices in government buildings.

What To Consider When Building A Sub-basement

basement room renovation, peeled of wall finishing, rebar on the flooring

Adding a sub-basement is not an overnight or DIY project. This rare home upgrade might be financially strenuous. However, homeowners should seek professional advice and assistance.

Proper survey and exact financial quotations are necessary when building a sub-basement. In some cases, a sub-basement might not have enough headroom or vertical space.

1. Topography

The topography of where your house is situated is vital. Different ground levels might hinder proper ventilation and light entry into your sub-basement.

A steep slope doesn't need a lot of excavation, unlike a moderate one. Especially, when adding on a sub-basement to an existing basement. A flat site will need a lot more work done on it.

Use the topography to help you get natural light in your sub-basement. You might not be able to install wide windows, but narrow ones will work.

2. Property Structure

You'll need to have a professional survey your property to understand if it's viable for any alterations. A strong foundation that can hold is a must because of excavations and continuous digging during the addition.

Adding a sub-basement needs heavy machinery that might destabilize your property. Professionals will advise the homeowner on how to reinforce the foundation of your property during and after construction.

3. Geology

Consult a local building official about the ground conditions. Before any digging can commence, the homeowner should be aware of the groundwater level and soil type of the vicinity. They must comprehend if these conditions will accommodate any changes.

If the ground is solid rock, it will be impossible to dig any further to construct a sub-basement. Experts will help you establish the geological conditions around your home to avoid uncertainties.

4. Waste Drainage

When adding a sub-basement that will be used as a bathroom, you must consider waste drainage. You must include plumbing if there isn't pre-existing plumbing or the plumbing won't accommodate the new changes in the sub-basement.

5. Access

Construction needs room and ample space around to facilitate machinery and laborers' movement. Ensure that you have enough area around your property for easy access.

Talk to your neighbors and let them know of any changes that you'll be making on your property. Ensure that you also have the proper documentation from the local authorities to avoid penalties.

6. Fire Escape

According to building laws and codes, a sub-basement must have a fire escape or an egress. The exit or egress can be via the main basement, but building codes demand an independent exit straight to the ground.

7. Additional Insulation, Height & Light

Depending on the purpose of your sub-basement, ensure that you have the necessary height of the sub-basement. Enough headroom will allow you to utilize a sub-basement as a bathroom, gym, or family room.

Ample light in a sub-basement is an important aspect. Where and when possible a sub-basement must have natural light for proper aeration. This will reduce mold formation.

Insulation shouldn't be put aside or forgotten. For your sub-basement to be fully functional and help you save energy, it must be insulated.

Dark basement of an abandoned building. Natural light.

Can You Extend The Basement Under Your Yard Past The House

Yes, you can. Extending the basement past the house into your yard can be done. Experts suggest doing the extension by digging in the yard towards the house or digging via the existing basement.

Before you start digging in the yard towards the house, have an expert survey. Digging up the yard might weaken it. Therefore, to avoid it from caving in, you'll need to reinforce the ground from below.

Alternatively, you can dig in your basement past your house past the boundaries of your house. A professional will help you understand which adjuments you will need concerning the existing foundation.

Experts with a good understanding of the soil structure will advise you accordingly on how best to start the basement's extension into the yard.

Can You Have A Two Story Basement

Absolutely! However, it's an expensive and time-consuming affair. Such a construction will include an additional foundation without adding any more square footage to the home.

Lately, story basements are not as popular for safety reasons. International Residencial Codes require that every basement should have an exit onto the surface other than interior stairs. Most newer homes are not able to meet up this regulation.

If the house is on a steep hill and the original plans include a two-story basement, then you can have a two-story basement by default. Two-story basements are commonly found in older homes and they house utility rooms. Here, you could find the boilers and furnaces that were used to heat water and the entire home.

Pros And Cons Of Sub-Basements

Many homeowners have basements but not many have sub-basements. Sub-basements can be add-ons. But, it's wiser and more cost-efficient to have a sub-basement built together with the rest of the house.

Sub-basements in residential buildings have their pros and cons.


  • They provide extra storage space in the home. A sub-basement might be used as a wine cellar, boiler, or furnace installation room.
  • They also reduce energy costs in the whole house. A well-insulated sub-basement lets you heat the house less.
  • A sub-basement might raise the value of your home, depending on the location. Homeowners with vast acreage can add a sub-basement to install an indoor swimming pool. Such homes can be put on the market for rent as holiday homes.


  • Costly to install in a pre-existing basement. To properly construct a sub-basement and insulate it, you'll need a lot of money. Spending a lot of money on space that might not be utilized, is a poor investment.
  • Not viable for newer homes. The International Building Codes have been reviewed in most states, and not all states will allow homeowners to add a sub-basement.

Sub-Basements in Newer Homes Vs. Older Homes

Building a sub-basement in an older home is much easier than in a newer home.

Older homes have dirt floors. They make it easier for homeowners to construct a sub-basement alongside the main basement. Here construction can be done by digging deeper without the use of heavy machinery.

Most newer homes have concrete basements. This makes it difficult to build a sub-basement without tampering with the foundation of the property.

Closing Remarks

Now that you can add a sub-basement to an existing one, don't let it be a place for cultivating cobwebs. Use it as a utility or storage room.

A sub-basement could increase the value of your property when properly constructed and insulated. You can remodel your home to include a sub-basement for extra space.

For information on different types of basements, read the following:

Can You Finish A Dirt Basement?

How To Vent A Gas Water Heater In A Basement

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