What factors affect the cost of a home addition?

For most homeowners, building an addition onto their home is a challenge. The delicate balancing act between adding both space and value to your house while also managing cost factors is enough to make many people think twice about an addition. 

Fortunately, the solution to building a cost-effective addition begins with a very simple step: understanding cost factors. At Lamont Bros., we emphasize client education as part of our design-build process, so you as the homeowner can make informed decisions for your home. When discussing an addition, one of the first things we address is the cost – and what can affect it.

This article will address five major factors that commonly affect the cost of a home addition. Keep in mind, these aren’t the only issues that determine the cost of an addition. Rather they are the ones that\ will have the most influence on your total project cost. These factors are:

  • Size
  • Design
  • Structure
  • Tie-in
  • Finish Material


Most people would rightly assume that the size of an addition is one of the most important factors to consider when estimating the cost. This is why the most common question people ask when considering an addition is, “How much does an addition cost per square foot?”

Unfortunately, that’s often a difficult question to answer. Every addition has a different set of needs, challenges, and specifications. These differences between projects make it nearly impossible to estimate a blanket cost per square foot.

As a general rule of thumb, you should expect two things to be true regarding the cost per square foot of an addition:

  1. The larger the addition, the higher the total cost. More square footage results in a higher final bill because for every square foot, you have to pay for materials and labor. 
  1. The larger the addition, the lower the cost per square foot. While this may sound counterintuitive, there are always some fixed costs when doing an addition. Whether it’s 100 square feet or 1,000, you’ll likely have to pay design fees, permitting fees, and labor costs that are simply unavoidable. Thus, the more square footage your addition has, the more those fixed costs spread out. 


If you want to keep costs low on an addition, simplicity is always going to save you. The more complex an addition, the more design work, labor, and materials it is likely to require. Keeping the addition’s design simple will ensure a manageable budget. 


Most design experts agree that the most cost-effective home shape is a rectangle. Four straight sides with 90-degree corners are the easiest to build and make the most efficient use of materials. If you’re looking to keep costs low on a home addition, you’ll do yourself a huge favor by using a rectangular footprint.

Inversely, you’ll likely incur additional costs by deviating to more non-standard shapes. 45-degree wall angles, circular rooms, and geometrically complex structures are all strong candidates for raising the cost of an addition.

Architectural Style

Ornate siding on Victorian home can be very expensive.

Another factor to consider in your design is the architectural style of the home. Oftentimes, homes with a clearly defined architectural style are more expensive to add on to. 

Every architectural style has its own set of design rules. These rules can affect everything from the materials used in construction to the pitch angle of your roof. In order to make sure an addition matches the existing home, the designers and builders must take special care to follow the architectural guidelines. More attention to detail means more time and expertise, which adds more to the total cost.

 For example, midcentury modern residential architecture is largely defined by straight, horizontal lines, large windows, and open floor plans. In order to achieve each of these, a designer must consider structural, aesthetic, and material limitations. Large windows tend to be substantially more expensive than more ‘standard size’ windows. The decorative exterior of a Victorian house cost significantly more than a more standard exterior treatment. 

Fortunately, most homes in the Portland Area tend to be relatively forgiving when it comes to architectural styles. This is because many of the homes in this area tend to fall into the category of transitional style. Transitional style homes are defined by a mixture of traditional and contemporary influences. With homes like these, there is much more flexibility when it comes to designing additions.

To learn more about the most popular architectural styles in the Portland Metro Area, read our article, “Portland Home Styles Guide.”


Our team at Lamont Bros. has built dozens of additions across the Portland area, and no two have ever been the same. When considering the cost factors of an addition’s structure, there are two main factors that can affect your cost.


2nd-story additions are among the most expensive.

An addition can be one level or several. It can go on the side of the house, above it, or even below. So, if you’re thinking about an addition, one of the first things to decide is how many levels you want it to be, and where. 

It should come as no surprise that a single-story, ground-level addition is usually the least expensive option. However, a two-story addition is often the most cost-effective option per square foot. 

It is important not to confuse a two-story addition with a second-story addition. Whereas a two-story addition is an addition that consists of two levels, a second-story addition is a single-level addition built on top of an existing one. Second story additions tend to be substantially more expensive than ground-level. This is because they often require structural reinforcement on the existing lower levels.

The title for most expensive, however, goes to basement additions. When adding a basement below an existing house, the house must first be lifted on jacks. The foundation must then be removed, the dirt dug out, and a new foundation installed. Basement additions are great solutions when you need more space and can’t build outward or upward. However, due to the intense design and construction process required to pull off a project like this, they are astronomically more expensive.


Another issue to consider when planning the structure of an addition is its foundation. This factor has the most impact on ground-level additions but can affect second-story and basement additions, as well. 

Most gound-level additions are built on a crawl-space foundation.

Traditionally, there are three options for foundation structures: slab, crawl space, and basement. A slab foundation is simply a flat slab of concrete on which the structure is built. These are often the least expensive foundation options. However, in climates like the Pacific Northwest, slab foundations are more prone to cracking due to the large temperature fluctuations throughout the year. 

Most homes in the Portland area have a crawl space foundation, which uses concrete walls to elevate the whole home’s structure above ground level. Crawl spaces require more labor and planning and are thus more expensive than slab foundations. 

The most expensive foundation option is a basement. Although they are more than twice the cost of a crawl space, basements add livable space to your home and increase its value.


Perhaps the most challenging factor in estimating the cost of an addition lies in how it connects to the main structure. Referred to as tie-in, the point of contact between the main home and the new addition presents a litany of obstacles.

The cost-effectiveness of an addition’s tie-in can be thought of as a ratio of addition square footage to linear tie-in footage.

Here’s what that means: every addition must tie into a home somewhere, usually at an exterior wall or set of walls. Sometimes, an addition ties into the entire side of a house. The builder must ensure that all points of contact between the home and the addition are secure and structurally sound. So, the more linear feet of addition connect to the home, the more expensive the tie-in will be.

Now, imagine two different additions. Addition 1 is a 10’ x 10’ room. Addition 2 extends the entire 25-foot side of a house by 4 feet. Both are about 100 square feet. The difference is, Addition 1 is a 10’ x 10’ rectangle, and addition 2 is  25’ x 4’ rectangle.

Which will be more expensive? Well, in order to tie into the home, Addition 1 needs 10 linear feet of tie-in. Addition 2, on the other hand, needs 25 linear feet of tie-in. As a result, Addition 1 will be much less expensive.

This isn’t to say that taking out an entire side of a home doesn’t ever make sense. Rather, the principle should be to make such a large tie-in worth the effort by maximizing the addition’s square footage.

An addition with a relatively simple tie-in. Note how the walls extend outward and the pitch angle of the roof is the same.

Finish Material

Though less consequential than the structural elements of an addition, finish materials can make a huge dent in your budget if you let them. Below are just a few examples of how your finish material can affect the cost of your addition.


Believe it or not, windows can be extremely expensive, sometimes adding tens of thousands of dollars to a project. In historic homes especially, windows can be a tricky factor.

While vinyl windows are a cost-effective, simple solution for most contemporary homes, some homeowners prefer wood or metal windows. If you’re trying to match your addition to the rest of your home and the windows are higher-end, you may end up spending more to match. 

In addition to materials, sizing can also affect your costs. Windows are typically offered in standard sizes, so if you have a window opening that requires a custom-sized window, you’ll likely pay a higher price for that, as well. 

So, if you’re looking to keep window costs low, vinyl frames and standard sizes are the way to go. However, you’ll want to consider how this will affect the visual design of your addition. To learn more about the different  types of windows, read this article


The cost of siding also depends on the material and type of siding you choose to use.  Vinyl lap siding will almost always be the least expensive option, as it is a cheap material and very easy to install. At Lamont Bros., we rarely install vinyl siding and typically recommend going with hardiplank instead. It’s much more durable, lasts longer, looks great, and is much better quality for the price.

Cedar lap or shakes tend to be more expensive, but also more traditional and aesthetically superior. The most expensive siding option is stone; while the material may not cost a lot, masonry requires extensive hours of skilled labor.

As with windows, the challenge with siding becomes working with what you already have. If you want your addition to match your home, most of the material choices have already been made.


Trim and finish on the interior of your addition can also affect your total cost. Drywall, flooring, baseboards, and paint are all items you should consider as potential cost factors for your addition. This is especially true if you want the interior finishes to match the rest of your home.

For more information on the challenges of making an addition that matches the rest of the house, read our article, “How to design an addition to match your home.”

Want to talk to a designer about a home addition?

If you’re like most homeowners considering a home addition, you probably want to make sure you get the most out of your money. For more information on the cost of additions, check out these case studies on the cost of additions in the Portland Area

If you’re ready to start planning an addition to your home, let us help you! Click the button below to schedule a free video call with one of our professional Design Consultants. They’ll help guide you through the process of remodeling your home, from start to finish.