Understanding Addition Cost per Square Foot: Case Studies and Analysis

Square footage is in high demand. Many homeowners are seeking to upsize their living space, whether it’s because their family is growing, they’re spending more time at home, or they simply want more space. With the current housing market making it difficult to buy a home at a reasonable price, some choose to simply add on to their current home rather than buying a new home. So how much does an addition cost per square foot?

Because additions come in many different shapes and sizes, this is a difficult question to answer. The best way to estimate the cost per square foot of a home addition is to examine a few real-world examples. As a remodeling firm that specializes in custom-designed home renovations, Lamont Bros. has performed dozens of home additions in the Portland area. 

In this article, we will explore the cost per square foot of a home addition by analyzing real-life case studies of several different types of home additions. We’ll discuss the cost per square foot of each addition, adjusted for inflation to 2022 dollars. This article will also provide a description of each project to give you a better understanding of how design can affect the cost.

The three standard types of additions you’ll read about are:

Is Cost Per Square Foot a Good Metric for an Addition?

Most contractors will tell you that thinking of additions in terms of cost per square foot isn’t typically a helpful approach. This is generally an accurate statement — there are simply too many factors that affect an addition’s cost beyond its square footage. Here are a few of them:

Static Construction Costs

No matter how large your addition is, there will always be a minimum amount of money you’ll end up paying to do an addition at all. This is the static cost. For this reason, smaller additions tend to be more expensive per square foot, because larger additions split out that static cost over more square feet. Think of it like the admission fee at the fair — you’ll pay for the individual rides later, but no matter how many rides you go on, you still have to pay the admission fee. The more rides you go on, the lower your cost per ride because the admission fee splits out among more rides.

Variable Cost Factors

The cost of an addition is also subject to several other factors outside of square footage. Of course, the size of the addition affects the cost, but so do other things like the quality of materials, the complexity of your design, and the state of your current home. Ease of access is also a major concern in urban areas, as houses that are close together tend to be more difficult to work on.

Overall Average Cost Per Square Foot of a Home Addition

The term “home addition” covers a wide range of projects with different sizes, needs, and ultimately, costs.  As a result, generalizing the cost of additions per square foot is a difficult and often unhelpful calculation. That said, if you still want to know the average cost of an addition per square foot, you can have it.

Over the past 3 years, Lamont Bros. has constructed 12 substantive additions in the Portland area. Of those, three were major outliers in terms of cost per square foot. Those were not factored into this average.  

Among the other 9, the average cost per square foot was $469.

Keep in mind that several factors will affect the cost per square foot of your own addition. It could be less than this average, or it could be more. Chances are, a single-story addition or a two-story addition will be around or below this average. Second-story additions, basement additions, and kitchen additions will likely be higher.

To learn more about what cost factors might affect your home addition, read our article, “What Factors Affect the Cost of a Home Addition?

Simple Single-Story Addition Cost Per Square Foot

The simplest and most common type of addition is a single-story addition. Usually, these are attached to the side of the house at ground level. Most homeowners use single-story additions to build a master suite, additional bedrooms, or more shared living space. Single-story additions tend to have the lowest base price due to their simplicity.

One of our most recent single-story additions was for a home in West Linn, OR. The homeowner, an avid pinball machine collector, wanted a game room for their collection. In terms of design, the addition was relatively simple; a rectangular 655 sq. ft. space with enough power outlets to run 30+ pinball machines at once. 

The total cost of this addition was $243,000, bringing the total cost per square foot to $371.

West Linn Single Story Addition

  • Level: Ground
  • Number of stories: 1
  • Foundation type: Crawl space
  • Shape: Rectangle
  • Room use: Game room
  • Added space: 655 sq. ft.
  • Cost: $243,000
  • Cost per sq. ft.: $371

Complex Single-Story Addition Cost Per Square Foot

Just because a single-story addition isn’t structurally complex in and of itself doesn’t mean it will always be less expensive. For example, take this home addition project, designed for a home in Southeast Portland.

The purpose of the addition was to add two new bedrooms to the home including a primary suite. Though its shape was a simple rectangle and the interior features were not especially complicated, there were two factors that dramatically increased the cost of this project.

The first factor was an old-growth douglas fir tree in the backyard. In Portland, trees are protected by local ordinances and construction cannot cause them significant harm. As a result, the design required extensive engineering to ensure that the foundation of the addition would not harm the tree. Ultimately, the design called for 11 vertical concrete footings for the home to be built on top of rather than a new crawl space or basement.

The second factor that influenced this addition’s cost was access. Due to the tight corridors between homes, it was difficult to move equipment and materials from the street to the back of the home where the addition was located. As a result, any excavation had to be done by hand or small machinery, which takes more time and costs more money.

All in all, this single-story, 313 sq. ft. addition came in at $251,000 for a cost per square foot of $802.

Southeast Portland Primary Suite Addition:

  • Level: Ground
  • Number of stories: 1
  • Foundation type: Custom-engineered concrete footings
  • Shape: Rectangle
  • Room use: Bedrooms
  • Added Space: 313 sq. ft.
  • Cost: $251,000
  • Cost per sq. ft.: $802

Second-Story Addition Cost Per Square Foot

Built over top of an existing level, a second-story addition tends to be towards the top in terms of cost. This is because second-story additions often require structural reinforcement on the lower levels, adding to both the design and building costs. They also tend to be less accessible than ground-level additions, and thus create more work for the construction team. 

Second-story additions typically make excellent space for additional bedrooms and/or bathrooms. Home offices, theaters, or multipurpose rooms are also common. They are a great solution in homes with limited options for outward expansion due to property boundaries. 

In 2021, we built a second-story addition for a family in the Cedar Mill neighborhood. The homeowners wanted to add a master suite above their existing living room/kitchen. Our final design included a 717 square foot addition with a master bedroom, master bathroom, walk-in closet, and private office space. 

Like most second-story additions, the challenges with this project were primarily structural – the first story on which the addition was built needed to be structurally reinforced before the addition went on. We also had to extend the chimney higher to compensate for the higher roof. In the end, the total cost for the master suite addition ended up being $365,000, for a cost per sq. ft. rate of $509.

Cedar Mill Second Story Addition

  • Level: Second
  • Number of Stories: 1 (Existing Lower)
  • Foundation type: Floor below
  • Shape: Rectangle
  • Room use: Master suite/office
  • Added space: 717 sq. ft.
  • Cost: $365,000
  • Cost per sq. ft.: $509

Two-Story Addition Cost Per Square Foot

Believe it or not, there’s a difference between a second-story addition and a two-story addition. With a two-story addition, the addition consists of two stories built at ground level. A second story addition involves building a second level on top of a pre-existing story. 

The primary advantage of a two-story addition is that it is an incredibly efficient use of space. You get twice as much square footage as you would with a single-story in the same footprint. For this reason, the cost per square foot of a two-story addition can sometimes be lower than that of a single-level addition. 

Two-story additions are incredibly versatile and can add virtually any kind of living space to a home. Homeowners often use them to add extra living space downstairs and more bedrooms upstairs. It’s also not uncommon to design a two-story addition to function as its own separate living space, like a mother-in-law suite. 

Recently, Lamont Bros. built a two-story addition in Beaverton. The homeowners wanted to add a bedroom, bathroom, and den to the downstairs and a multi-use bonus room with a balcony to the upstairs. We built a 998 square foot addition for a total project price of $347,000. At a cost per square foot of $348, this two-story addition was less expensive per square foot than the single-level addition even though the total job cost was more.

Beaverton Two-Story Addition

  • Level: Ground
  • Number of Stories: 2
  • Foundation type: Crawl space
  • Shape: Rectangle
  • Room use: Bedroom, bathroom, den, multi-purpose
  • Added space: 998 sq. ft.
  • Cost: $347,000
  • Cost per sq. ft.: $348

Basement & Kitchen Addition Cost Per Square Foot

In addition to these three main types of additions, we also see a fair amount of additions involving basement and kitchen space. These types of additions tend to be much more expensive and less predictable than those mentioned above.

Basement Additions

A typical basement addition involves adding a basement to a house. This means digging out the foundation and building downward rather than up. Due to the cost of excavation and concrete, basement additions are extremely expensive. They often involve specialized digging equipment and difficult permitting procedures. Because building underneath a home can present many different challenges, it is nearly impossible to estimate the general cost of basement additions.

Kitchen Additions

It’s also common for homeowners to add space to their home while also remodeling their kitchen. These kitchen additions often have a very high price per square foot simply because of the cost of the kitchen itself. 

For example consider 2 kitchen addition projects by Lamont Bros. The first was a $228,000 kitchen remodel in 2020 that added 109 square feet to the client’s house. Due to the cost of the kitchen remodel, the cost per added square foot was $2,089. The second project in 2022 cost $341,000 and added 224 square feet for a cost per square foot of $1,522.

However, it is important to note that most of the project budget for a kitchen addition doesn’t actually come from adding space itself. Rather, the largest cost items are the cabinets, countertops, and appliances that are all standard features in any kitchen remodel.

Like basement remodels, it is difficult to accurately calculate the cost per square foot of a kitchen addition because the cost of the actual kitchen remodel varies so greatly from project to project. 

Thinking about an addition for your own home?

Now that you have a strong understanding of what your home addition might cost per square foot, take the next step. To learn more about the cost of a home addition, check out our article, “How Much Does a Home Addition Cost in Portland?

If you’re ready to start making big decisions for your home, let us help you! Lamont Bros. has guided dozens of Portland homeowners through the process of designing and building their dream addition. Click the button below to schedule a free video call with one of our professional design consultants.