How to Build a Basement Under an Existing House | 18 DIY Steps


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Building a basement under an existing house can be complex and labor-intensive. This guide will teach you how to build a basement under an existing house.

It is often necessary to have a professional contractor doing the work, as the process may involve excavating and installing complex structural supports. This is what you can do to build a basement under an existing house:

  • Assess your current home for structural and aesthetic integrity
  • Research local regulations and building codes
  • Get permission from the local regulations.
  • Calculate the size and shape of your basement
  • Check for underground utilities and drainage
  • Develop a plan for the basement foundation.
  • Remove any existing obstacles
  • Establish the boundaries of the basement
  • Dig the basement foundation
  • Install footings and pillars
  • Install support beams
  • Install the flooring 
  • Pour the concrete walls
  • Install basement windows
  • Install the staircase 
  • Install plumbing, electrical, and HVAC systems
  • Install the insulation
  • Finishing the basement with drywall, flooring, and appropriate fixtures

However, in this Home Affluence post, we will explore the steps necessary to build a basement under an existing house and discuss the essential safety precautions to take during the project. 

How To Build a Basement Under an Existing House

Assess your current home for structural and aesthetic integrity

Before you begin building a basement under your existing house, assessing your home’s structural and aesthetic integrity is crucial.

Assess your current home for structural and aesthetic integrity

A thorough inspection of the foundation, walls, and roof can help detect any existing structural problems that may need to address before beginning construction. Additionally, it is crucial to ensure that the home’s aesthetic will stay intact during the basement construction process. 

Make sure to go over any plans proposed with a fine-toothed comb and consider any potential challenges that may arise during the construction process.

Research local regulations and building codes

Before you start any construction, it is important to research local regulations and building codes that apply to your project.

You may have to obtain a permit or have plans approved by your local municipality or building department before you can start building your basement.

Research local regulations and building codes

Different localities may have other requirements, so check with your local government to determine what do require. Make sure that you follow all guidelines to ensure a legally-compliant basement.

Get permission from the local regulations.

Before you begin construction on your existing house, getting permission from the local regulations in your area is vital.

Depending on where you live, you may need to submit plans and obtain a permit from your local building authority. 

In some cases, you may also need to prove that the construction of a basement will not cause any harm to the existing building or surrounding area.

Get permission from the local regulations.

Additionally, it is essential to check for any zoning regulations in your area that may affect your basement construction. 

Taking the time to seek the necessary permission and comply with all local regulations will ensure that your project runs smoothly and do complete safely.

Calculate the size and shape of your basement.

One of the most critical steps in building a basement under an existing house is to calculate the size and shape of your basement.

Calculate the size and shape of your basement.

It does determine the number of materials, labor, and cost you need for the project. To get accurate measurements, measure the basement from the inside and not the outside. 

Measure the width and length of the walls, the ceiling height, and the floor’s depth. Once you have those measurements, use either a calculator or an online tool to determine the size of your basement. 

Also, measure any windows, doors, and other openings you plan to include in the design. It does help you to plan the layout of your basement.

Check for underground utilities and drainage.

One of the most critical steps when building a basement under an existing house is to check for underground utilities and drainage.

Check for underground utilities and drainage.

It does ensure that there will be no disruption to the existing utilities, drainage, or sewerage system as a result of the construction.

It does also help ensure that all building codes and regulations meet and that the basement does build safely and securely.

Develop a plan for the basement foundation.

Before you can start building your basement, you need to develop a plan for the basement’s foundation. 

Choosing a foundation type is necessary because there are several basement foundations.

It includes identifying and assessing the soil conditions underneath the house, determining the type and size of footings needed, and calculating the size of the foundation wall

Develop a plan for the basement foundation.

If your plans include a basement with a full cellar, you will also need to consider the size and type of support beams and columns required to support the weight of the house above. 

It can be a complicated process, and it is best to consult a professional engineer to ensure your plans are accurate and safe.

Remove any existing obstacles.

Before you start building your basement, you must remove any existing obstacles. It includes demolishing any existing structures in the space you’re working with.

Remove any existing obstacles.

You’ll also need to check for underground obstacles like utility lines or pipes. If there are any, you’ll need to either reroute them or hire a professional to do it for you. 

Additionally, you may need to have the area inspected by a professional before you begin building a basement under an existing house.

Establish the boundaries of the basement.

Before you begin any construction work, it is essential to establish the boundaries of the basement.

To do this, you will need to measure the basement area and create a diagram of your home’s foundation.

Establish the boundaries of the basement.

It does help you ensure that the basement is within the boundaries of the existing house.

Additionally, you must confirm that the basement will not interfere with any utility lines running through the area.

This step is critical, as it will help you avoid any costly issues that could arise from not following local building regulations.

Dig the basement foundation.

Digging the basement foundation is a critical step in construction and requires precision and expertise. 

Dig the basement foundation.

It is vital to ensure the foundation is level and has the correct depth to support the structure of the building.

The excavation process should account for local soil characteristics and any underground utilities. It should be large enough to support the foundation walls, footings, and other structural components.

It would help if you cleared the area of any debris and vegetation, and the soil should backfill and compact.

Necessary to follow safety protocols while digging the basement foundation, such as providing adequate ventilation, testing for hazardous gases, and using protective gear. 

Additionally, the contractor should adhere to local building codes, regulations, and permits before beginning work. 

Properly digging the basement foundation is essential for building a safe and secure structure.

Install footings and pillars.

Installing footings and pillars is critical for any basement project involving a building’s foundation. Footings provide the necessary base for the pillars, which provide the primary structural support for the installation. 

Footings can be constructed from concrete or masonry materials, depending on the structure’s needs.

Depending on the soil conditions, footings may also need to reinforce with steel rebar.

Install footings and pillars

During installation, footings should be set according to the building plans and checked for levelness and plumb. Once in place, the pillars can be set according to the same constraints, with mortar or concrete being used to secure them.

Once both footings and pillars do correctly install, the building’s foundation can be considered complete.

Install support beams

Installing support beams for your new basement is crucial in various construction projects, as they provide the necessary structural support to ensure the structure is safe.

Install support beams

Use high-grade materials and follow manufacturer specifications closely.

Depending on the project, the support beams may be installed during the initial construction or used to reinforce existing structures.

The installation process typically involves the following:

  • Positioning the beams in the designated locations.
  • Attaching them to the main framework of the structure.
  • Securing them with bolts and other appropriate fasteners.

Professional installation of support beams is essential to ensure the safety of the structure and the people who use it.

Install the flooring

Installing flooring can be daunting, but with proper preparation, the process can be completed efficiently and with great results.

First, it is vital to measure the space accurately, ensuring that you have the correct amount of flooring material.

Once the materials have to do purchase, it is essential to accommodate any irregularities in the space, such as door frames or existing fixtures.

Before beginning installation, it is also vital to ensure the flooring material is clean and debris-free.

Install the flooring

Once the preparation is complete, the actual installation can begin. Depending on the type of flooring material, installation methods may vary. 

For example, hardwood flooring can install using either a floating or nail-down method. 

If using a floating method for your new basement, it is crucial to ensure that the planks fit closely together and that the floors are level.

Pour the concrete walls.

The construction team is getting ready to pour the concrete walls for the new basement. They have gone through all the necessary steps to ensure that the structure does appropriate support and that the walls are level and even. 

Pour the concrete walls.

First, the team laid out the forms and applied reinforcing steel to provide strength and rigidity. 

Then, the concrete mix was prepared, ensuring the proper ratio of cement, water, aggregates, and admixtures.

The concrete was delivered to the formwork continuously to ensure an even pour. 

Once the concrete did pour, the team used a bull float to level the walls and ensure a smooth finish. 

The concrete does cure, the forms can remove, and the walls will be ready to finish.

Install basement windows

Installing basement windows can be a great way to add natural light and ventilation to basements and create an additional emergency exit.

It is crucial to consider the climate and local building codes when selecting the basement window’s size, type, and material.

Install basement windows

It does recommend hiring a professional contractor to cut openings in the basement walls and adequately install the windows. 

Before installation, the contractor should also check for structural issues in the basement walls, such as water damage. 

Once the windows have to install, weatherproofing the window openings should be completed to ensure that water damage does not occur. 

Installing basement windows can make your basement healthier, more comfortable, and safer.

Install the staircase

Installing a staircase for a basement is an integral part of any home improvement project. It requires careful planning and preparation before beginning the work:

Install the staircase
  • It is crucial to determine the desired staircase type and select the appropriate construction material.
  • The area must determine the staircase’s location and inspect for potential obstructions or hazards. It is also essential to measure the extent and calculate the size of the stairs.
  • The installation begins, including framing the staircase, attaching the risers and treads, and completing any other necessary steps.

With proper preparation and planning, a staircase can install safely and efficiently, ensuring years of enjoyment and reliable service.

Install plumbing, electrical, and HVAC systems.

Installing plumbing, electrical, and HVAC systems is essential to any basement construction or remodeling project.

Install plumbing, electrical, and HVAC systems.

It is essential to hire a professional to ensure that these systems are properly installed and functioning correctly. A professional can determine the correct size and type of equipment needed to suit the project and ensure proper installation and connection. 

They will also ensure that all necessary codes and regulations are adhered to. Additionally, a professional can guide future maintenance and repair of these systems. 

Install the insulation

Installing insulation is an essential step in creating an energy-efficient home. Insulation can help reduce energy bills, increase comfort, and create a healthier indoor environment.

Choosing the right type of insulation for your basement is vital to ensure it does install correctly. To begin, measure the space to be insulated and select the appropriate insulation material. 

Install the insulation

Next, cut the insulation to the correct size and place it into the space. Secure the insulation with fasteners or adhesive as needed. 

Finally, inspect the insulation for any gaps or voids and seal them accordingly. With proper installation, insulation can help create an energy-efficient and comfortable home.

Finishing the basement with drywall, flooring, and appropriate fixtures

Finishing the basement with drywall, flooring, and appropriate fixtures is a great way to add value to your basement. 

Finishing the basement with drywall, flooring, and appropriate fixtures

We recommend that you start by properly insulating the walls and ceilings. It does provide a sound barrier against noise and helps regulate the space’s temperature.

Once the insulation is in place, you can hang drywall, apply a finish, and add flooring.

Frequently Asked Questions

What is the cost of building a basement?

The cost to add a basement depends on the ground’s current situation and what services need to fix it.

The price will depend on these four factors:

  • Structure (the shell and core)
  • Waterproofing depends on how many water pumps and sump chambers are needed.
  • Fit-out
  • Maintenance

The following table will show the average estimated price for building a basement:

Average cost$10-$100 per square foot
National Average$8423 per square foot
Lowest Price$1200 per square foot
Highest Price$25000 per square foot

How much does it cost to dig a basement?

The following table will represent the overall basement to dig out costs researched prices:

Average cost$20,000-$150,000
Partial installation$50,000

Does adding a basement add value?

Adding a basement to a house amazingly increases space, although if you take proper care of it, it will increase the house’s value. But if the basement is completely flooded, it may reduce a home’s value.

Can you add to an existing basement?

Yes, it is possible to execute and build a basement onto an existing house. Basement building will involve excavating a small portion or digging under the slab to create an entirely new area, depending upon the entire crawl space. Sometimes due to rats, you get into trouble in your crawl space. Use these techniques to get rid of rats in a crawl space.

How much does it cost to frame a 1000 sq. ft. basement?

If we discuss a 1000sq ft. basement, the same price table is mentioned below. But it may vary according to your area or suburb.

SizeDIY CostProfessional cost
Small(<700 sq ft.)$5,500$15,000
Average 700-1000 sq ft$8,000$18,500
Large 1000+ sq ft$15,000$35,000

This is the estimated size for 1000 sq. ft. But it still may vary according to your area or according to the value of your house.

Is a basement or 2nd story cheaper?

It depends upon the following factors:

  • Comparative design
  • How much limestone, granite, or other bedrock is under your topsoil?
  • For the second story, you will need stairs, a landing, and corridor, and storage on the 2nd floor to access the bedroom, bath, and storage and make the 2nd floor usable.
  • While the basement also needs all of these things depending on your requirement, like making it a walkthrough, you want to make it a storage room, or whatever else you want to make it.
  • But if we see a simple estimate, then the basement is cheaper than 2nd story as we can use the basement for any purpose we need. But comparatively, the 2nd floor needs more attention like a good ceiling, good bedrooms for stay, bathroom.

What is the cheapest way to finish a basement?

The cheapest way to finish a basement is to precise in your priorities.

  • When working on the ceiling, paint it with a warm and saturated hue; it still looks good in low lighting.
  • You can make a floor with concrete or tiles. It’s a less expensive way. Vinyl flooring also proves itself budget-friendly and water-friendly.
  • The next thing is to dress up your windows. The best option is to fix the rod high and wide to hang ceiling-to-floor panels to make it look bigger.
  • The next thing is to lighten up space; instead of reviving the wall with pot lights, you can put lamps in a row that will give you a fantastic view and boost light in the room.

Expert Opinion

In conclusion, building a basement under an existing house is challenging but rewarding. There are many steps to consider, from excavation to installing a new foundation. 

Working with a professional will help ensure that your basement is up to code and meets all safety regulations.

We suggest you choose waterproof, mold-resistant materials for the flooring, such as tile or linoleum. 

Finally, you can add lighting, outlets, and switches. With these simple steps, you will have a finished basement that adds value to your home.

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