How to Add a Room to a Finished Basement

Unless you’ve been lucky enough to design and build your own custom home, the odds are that you bought a home that most closely matches your needs but not perfectly. Homeowners often consider additions and remodel to help make their house a home. Above-ground construction can be fairly straightforward, but what about below ground?

The complexity of adding a room to a finished basement depends on whether you need to add an addition to your basement or to modify the layout of an existing basement. After checking to make sure you can meet all code requirements for the type of space you want to add, you can either modify your finished basement yourself or hire someone to excavate space for another room.

To help you understand what options are available when remodeling your basement, we’ve compiled some information for additions, remodels, codes, and practical advice for your next project.

Modifying the Layout of a Finished Basement

The easiest way to add a room to your basement is to modify the layout of the interior walls of your basement. While you may not gain additional area to add to the appraised value of your home, you may gain improved functionality of your home.

You may be able to move an existing wall yourself without having to apply and get approval for a building permit from your city’s building department or even hiring a contractor. However, most jurisdictions require you to provide drawings for approval to get a building permit if you are modifying an existing electrical circuit, plumbing, or ductwork from your HVAC system, and especially if you are modifying an existing load-bearing wall.

If you are simply adding a new wall, you may be able to do it yourself. You can use your new room as a living space, den, game room, storage space, or office, or any other use as long as people do not sleep there.

Building codes in the United States do not allow bedrooms in basements unless there are two ways to get out. Usually, the two ways out include a window that’s big enough and low enough for an occupant to escape in addition to the bedroom door. The requirements will vary depending on the local codes in your area, so you should check with the building department or ask advice from a contractor or architect in your area.

To move an existing wall, first, check if there are any receptacles in the wall. If there are, you’ll need to get a permit and a licensed electrician involved. The same will go for HVAC or plumbing. While you could do it yourself, unpermitted improvements to your home will not add value to your home’s appraised value and could hurt your ability to resell your home. Although it’s rare, if a city inspector was tipped off to unpermitted improvements, you could be fined and required to make sure the improvements are code compliant.

If building permits need to be obtained, you should find a licensed architect or contractor in the area to develop architectural plans. They can help relocate a load-bearing wall, or otherwise re-support the weight of the floors and walls above. We’ll discuss later in this article how to hire an architect or contractor to help you with this process.

How to Frame an Interior Wall: 

Adding a new interior wall is a fairly straightforward process that most people will be capable of doing. Only a few tools are needed, such as a saw, a drill, measuring tape, and a hammer.

Here are a few easy steps to framing an interior wall:

1. Measure the length of the wall you want to add carefully.

2. You may need to cut finish material away to have a secure attachment to the floor or ceiling structural members. It may be helpful to outline where the new wall will go so that you can use a box knife or saw to cut away the sheetrock or carpet. Also, trim the sheetrock from any perpendicular walls your new wall will attach to.

3. If your new wall does not align with a stud in your adjacent wall, add nailing blocks between the studs. Nailing blocks are just short pieces of lumber that can be nailed into the bottom and top of the soleplate (bottom board) and top plate (top board) of the existing wall, allowing you to attach the new wall.

4. Cut your top plate and sole plates to your desired length, then use a pencil to mark where your studs will go, typically 16” “on center.” On center means that the center of the lumber aligns with your mark, as opposed to one outside edge of the lumber.

5. Nail the new top plate to your ceiling framing and sole plate to your floor framing. Make sure that they are perfectly aligned. If they aren’t aligned, when you add your interior finishes, such as sheetrock, you will see the material bend or warp to attach to the framing.

An easy way to do this is to use a plumb bob, which is essentially just a weighted string that hangs perfectly straight down from the top plate to where the soleplate should be attached. Use a pencil to mark dots where the string touches, then use a chalk line or long ruler to draw straight lines between the dots.

6. When framing walls for new construction, often the walls are framed outside, on the ground, laying down, so that the studs can be nailed into the top plate and bottom plate, then lifted into place in the home.

When building a new wall in an existing home, you won’t have the room to lift the wall into place. Therefore, you need to use stud framing clips to secure the studs at each mark you previously made. You may be able to “toenail” the studs to the plates, where nails are nailed in at an angle through the stud, but this type of attachment is less secure.

7. Double up the studs at the ends, corners, or where the new wall butts into an adjacent wall. If the wall has a corner, frame with two full-length studs with blocks in between them.

8. Once the wall is framed, you may add the finish material. Sheetrock is the most common material. With a partner, align the sheets of sheetrock and screw into place, beginning with the outside corner or edge. Fill the cracks in between the sheetrock and screw holes with “mud.” Premixed mud can be easily applied with a spackling knife.

9. Apply tape to the mudded cracks between the drywall, using the knife to smooth the tape into the mud. Make sure to also mud and tape the corners of your walls as well.

10. Since mud will shrink as it dries, you may need to add multiple coats to fill the cracks.

11. Use sandpaper, a palm sander, or sanding blocks to buff out any rough or raised mud. You may be able to use a very lightly dampened towel to brush off any dust on the walls.

12. Prime the walls with high-quality primer paint. The fresh sheetrock will absorb a lot of the paint, so consider if you may need to apply a second coat of primer after the first coat has dried.

13. Finally, you can paint your walls your desired color, add molding or baseboards for a nice finished look.

Expanding an Existing Basement

If you want to add an additional finished floor area to your home, you will need to hire a contractor or architect to help you develop the plans for approval by the local building department. Most residential homes with basements use the basement walls as a foundation for the exterior walls of the home above.

Moving, cutting holes in, or otherwise modifying these existing walls requires an understanding of the structural loads and detailing of the waterproofing of the envelope of your home.

Most commonly, basement walls are foundation walls that extend from the exterior walls of the home above down into the earth below. These foundation walls are typically concrete or concrete masonry units (CMUs), otherwise known as cinder blocks. The floor of the basement is typically slab on grade, which is concrete poured right on top of the ground below. Sometimes gravel, vapor barriers, or compacted sand may have been used underneath the slab.

The new addition will first need to be excavated. There could be a substantial cost to removing all of the existing dirt for the area you want to add.

To provide a proper foundation for your basement, the contractor may need to dig further down into the earth and fill the dirt back with gravel or compacted soil to make sure that the addition will not settle or shift substantially from the existing house. The most common source of cracks, water penetration, or building failures comes from the joining of an existing structure to a new structure.

Types of Foundation Walls

You will need to provide new foundation walls for your new addition. If your existing basement does not already match the footprint of the house above, you may decide to add new foundation walls underneath the exterior walls of your house, replacing any existing crawl space or post foundation.

If you are expanding your basement beyond the footprint of your house, you should consider making sure the foundation walls could be a base for a new above-grade addition in the future.

Here are the most common types of walls found in finished basements:

Poured Concrete Basement

The most common type of basement walls, this type of wall starts with a concrete footing. The contractor will excavate the entire footprint of the basement, plus a few feet of extra dirt around the perimeter. The footing is a wide strip of concrete around the perimeter of the basement that helps spread the weight of the walls above through the earth, keeping the house from sinking into the ground.

On top of the footing, the contractor will erect forms to pour concrete into for the basement walls. Once the concrete has cured, the contractor will remove the forms, backfill around the basement with gravel or dirt, depending on the water table levels or soil quality. The rest of the home will be built on top of the poured concrete walls.

Block or Masonry Walls

Block or masonry walls can be much more cost-effective than poured concrete and faster the build. However, they aren’t as strong as poured concrete.

With this type of wall, once the contractor has poured the concrete footings, they would then start laying the cinder blocks or bricks, grouting in between each brick. Sometimes, they may add reinforcing steel, or rebar, into the masonry to help strengthen the wall.

Precast Panel Walls

As a hybrid between concrete and masonry, precast panels are poured in another location, or somewhere on the construction site. Once they are cured, they can be erected into place in the basement. This option can help speed up construction time but may be more costly than either other option.

Regardless of the foundation wall type, it may be required or highly recommended to install a vapor barrier and insulation to keep the coolness and moisture from the earth out of your basement.

Types of Basement Floors

You will also need to add a new floor to your addition. The existing floor in your basement should be extended into your new addition. Depending on the age of your house, building codes may have changed, requiring additional design elements that the existing home may not have had.

This could include vapor barriers, which keep the moisture in the ground from coming up through porous concrete into the basement or drainage systems like french drains, which keep rainwater and groundwater from seeping through basement walls. Most basement floors will be poured concrete on top of gravel or sand. Make sure that a vapor barrier is provided to keep moisture out of your basement.

There are a few options for different types of basement floors, depending on the type of soil or water table levels in your area.

1. Concrete Slab: The most common type of basement floor is a concrete slab. After the contractor has built the foundation walls, they can backfill the bottom of the basement with dirt, sand, or gravel. The ground can be a source of moisture underneath your home, so in high moisture areas, you may need to install a french drain around the perimeter of your basement for built-up moisture to drain away from your home.
2. Framed floor: Uncommon in basements, many homes without basements may have a framed floor, with a crawl space underneath, or even just a small gap between the earth and bottom of the floor. The framing can be from steel, which is typically very expensive, or dimensional lumber. However, lumber can be prone to moisture damage.

How to Modify an Existing Basement Wall

If you want to connect your existing basement to the added room in your basement, you will need to cut an opening in your existing basement wall. Since foundation walls are typically concrete or masonry, removing a portion of the wall will require more robust tools than what a homeowner typically has. Since the foundation walls support the house overhead, the weight will need to be redistributed and supported to prevent sagging or cracking overhead.

Your contractor will be able to use concrete cutting saws to cut the new opening. They may need to add temporary supports to bear loads of the home until the new permanent structure is put into place.

A structural engineer will need to select and size the new structural elements properly. They may select a steel beam to span the gap over the new opening and add columns for the beam to bear on. The existing footings may be adequate for the new, increased loads on a smaller footprint, but if not, the footings may need to be reinforced.

To minimize the cost impact of the new structural elements, it’s recommended to make the new opening as small as possible, such as a single door or hallway width. If the opening is wider, for example, if you’re trying to create an open floor plan basement, the structural engineer may recommend intermittent columns and deeper, bigger steel beams to support all the overhead loads.

Can You Add a Room to a Condo?

Condominiums, otherwise known as condos, is an individually owned unit within a larger structure or complex. Condos could be known as apartments within an apartment complex, with the key distinction that apartments are leased or rented to a renter through a rent or lease agreement, as opposed to being owned by an individual.

Typically, the owner of a condo will need to pay homeowner association (HOA) fees that pay for common use spaces, such as pools, complex gyms, landscaping, or even shared utilities like water or trash.

Unfortunately, any and every remodeling project that’s undertaken in a condo will likely need to be confirmed and approved by the HOA. You may read through your HOA agreement to verify if you can add interior walls, paint, or replace appliances like stoves or AC units, without needing HOA approval. Even if the HOA doesn’t have limitations, you still have to comply with building codes and jurisdictions, as described above.

The type of condo you own will impact what type of addition you may be able to add or how you modify an existing layout.

If your condo shares multiple walls, floors, or ceilings with neighbors, you likely won’t be looking at addition, but rather an interior renovation. While you might think as long as you own the home, you can modify the interior as much as you like, some HOAs will want to approve any plans before them being implemented.

If you don’t follow the rules and regulations in your HOA agreement, you could face heavy fines or lawsuits, even for something as minor as adding or moving a wall.

How To Get a Building Permit to Add a Room

Even the most confident DIYers might feel intimidated by having to obtain a building permit and pass inspections. Most would likely wish they could just do it themselves without having to deal with all the bureaucracy. Building codes and building departments help keep our cities and neighborhoods safe, help preventing fires, injuries, or deaths due to unsafe building practices.

Most building departments are very friendly. Some departments might be busier than others, like large cities that review multiple, high rise, commercial projects a month versus small towns that get just a few building permit applications a year.

A quick phone call might help you understand what your local building department will require regarding forms, fees, and even provide a checklist of what you need to show on drawings.

Most building plans and drawings need to be prepared by someone that has a license to do so. A contractor might have all the licenses necessary or could hire someone, called a sub-consultant or sub-contractor, that has specialty licenses.

These specialty licenses might be required for structural modifications, like cutting and supporting part of a building foundation or an electrical engineer or electrician that can provide the modifications to your electrical circuits or even the addition of a new electrical panel board.

Your design and construction team should provide the building plans that clearly show how the house is designed and what is needed to build it. You should be able to guide the team with recommendations and preferences, such as how big or small you want the rooms and especially with the types of lighting, plumbing fixtures, or finishes that you want.

Once the drawings are complete, you or your contractor will submit the plans to the city for approval, along with any supplemental forms or application fees. The city will review the plans, either with you in an “over the counter” approval process, or they will hold on to the plans to review at their own pace. In both scenarios, they may make comments or request changes.

If so, you and your contractor will implement those changes, resubmit, and get final approval. Once approved, your or the contractor will be able to “pull” a building permit.

During construction, the contractor may need to call for the city to inspect portions of the project before they are hidden or covered up, like foundation walls before backfilling, or electrical work before the sheetrock is installed. The inspector will need to come on the site, review the progress, and may make comments about the installation to help the contractor meet building codes.

Some cities may allow the owner to do some of the construction themselves, but make sure you know what items they require a licensed professional to do. These items may include the construction of the foundation walls, rough-in of any plumbing fixtures, and installation of electrical systems. Some cities may also require the contractor to be responsible for closing up the floor, walls, and roof, leaving the interior finishes up to the owner to finish.

When selecting your design and construction team, here are some recommendations:

1. Ask friends and family who they have used in the past and whether they recommend the company or not. Online reviews sites can be helpful but always use discretion when reading online reviews.
2. When reaching out to an architect, structural engineer, or contractor, ask for example projects that they have recently completed that are similar to your project. If these projects are local, consider driving by discreetly to verify the quality of work yourself. Just don’t disturb the residents or invade their privacy.
3. Request quotes from several companies to get a range of comparisons. If you have never hired a contractor before, you may find it difficult to know if the prices are reasonable or if the quote package is professional and complete. Consider getting up to three different quotes from three different companies.
4. Interview the company over the phone or in person. Ask them questions about their quote if anything seems incomplete or unclear. Gauge their attitude and personality to see if you would enjoy working with them throughout the timeframe of your project.
5. Once you select the company, you want to work for, request a contract that clearly outlines what the company is being hired to do, how much they will get paid, and how often they will get paid. You may even ask a lawyer to review the contract if you are unfamiliar with legal terms like indemnity, mediation, and arbitration.
6. If the contract terms are unreasonable, or if the contract is vague or unclear, ask for revisions. If the company is unwilling to revise or compromise on the contract terms, do not be afraid to find another company.

Ultimately, adding an addition to your finished basement should be a fun and exciting project to help your house feel more like home. If you know what you should look out for, your project should be completed on schedule, on budget, and look like what you imagined.