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How Much Does a Home Addition Cost in Boise?

Preparing the Budget to Build a New Addition

Boise home addition

The ultimate way to add large amounts of fully usable space to your home and enhance your house's resale value is to build a home addition. The first question on every homeowner's mind is likely to be how much does an addition cost? It's never a simple answer because of the many variables but determining the average cost of the project is one of the necessary first steps in planning a home addition.

Expect the typical cost of a home addition in Boise to range between $250 per sq ft and $400 per sq ft. However, the reality is that the cost could rise past $500 per sq ft depending on many factors, such as material, labor, the size of the addition, and the type of the addition.

How Much Does a Home Addition Cost?

Average Cost

$300 per square foot

High Cost

$800 per square foot

Low Cost

$150 per square foot

Average Cost

The average cost of a home addition in Boise is around $300 per square foot, or for a 500 square foot addition around $150,000. Adding a bathroom or kitchen to that space involves plumbing, more electrical work, cabinetry/countertops, more tile, and more finish work on average which will bring up the total average considerably depending on the size and amenities of the bathroom or kitchen.

Low Cost

Some lower-cost home additions with few or no expensive services that require extensive electrical and plumbing work and builder-grade materials include small bump-outs, porches and decks, and dormers. Additionally room additions that do not add extra electrical or plumbing can be significantly less than kitchen & bathroom additions.

High Cost

Higher-cost home additions with extensive electrical and plumbing work and higher-grade materials include master suits, in-law suites, bathrooms, and kitchens which range in average costs from $300 to over $500 per square foot.

How Can an Addition Cost as Much or More then a New Build?

There are multiple variables that can cause an addition to cost more per square foot then a new build. First is the state of the existing plumbing, electrical, and HVAC systems. If you have an older home you might need to perform a complete update to all three systems in order to meet current code since building the addition could mandate that the existing house is brought up to code. Additionally, your existing mechanical systems may not be able to handle the additional load from the addition – say for example you are out of room on your electrical panel, then you will need to upgrade your electrical service. Or maybe your air handler will not be able to properly heat and cool the additional space – then you’ll need to update your entire HVAC system or install a separate heating and cooling system for the addition such as a mini split.

Second, design, project management, and labor for additions often takes longer and is more complicated then a new build. When building an addition, odds are you’re creating a completely custom one of a kind space. This means that there will be time (and thus Money) spent by the designer, general contractor and their trade partners to figure out how to actually make the construction plans work. Additionally, if you’re living in the home at the time of the remodel, your General Contractor is also going to spend a considerable amount of time taking extra care for site clean up and management to keep your home in much cleaner shape then the average new construction site. They may also opt to work with trade partners that are cleaner and present better on average, which also comes with a price.

Factors That Increase Home Addition Costs


The size of the addition is a major determinant of the total cost of the addition. The average cost of an addition, including materials and labor, can cost between $250 to $800 per square foot.

Type of Addition

The purpose of the addition often changes its price. A room addition, for example, will cost less than an addition that will operate as a separate apartment. Unlike full-size additions, room additions do not have expensive services such as full or even half bathrooms and they have decreased heating and cooling needs, fewer windows and doors, and more straightforward electrical work.

Electrical Work

Electrical work is necessary for even the most basic type of home addition. Electrical codes provide minimum requirements for lighting and outlets. When home additions become large and more complicated, the electrical cost increases exponentially.

Installing specialty spaces like a home theater, home office, or gym can drive up the cost of the home addition. Added features to other rooms such as electric radiant heating under a bathroom floor, hot water heater, electric dryer, and electric range may also drive up the price.

Plumbing Work

The more plumbing work, the higher the overall cost of the addition, especially if you’re tying into an existing structure with a slab on grade foundation which requires opening up the concrete to get access to the existing plumbing.. Plumbing work has to be tied into the existing plumbing in the main house.


Adding only a bathroom to your home can cost about $500 per square foot. When you start to add additional plumbing fixtures and custom electrical configurations, the cost will rise. Other materials like porcelain tile or natural stone, cabinets, and solid surface countertops are expensive per-square-foot materials & installation cost not typically found in other parts of the home.


The area or location where you want the addition to be built can also affect the cost. For example, will the addition have easy access to current features like HVAC systems, electrical lines, and plumbing? If so, that could bring the costs down a bit.


Any size addition requires a lot of labor. Here are some average costs to give you a general idea of what to expect depending on what type of addition you want.

General contractor: Fee is typically 20% to 35% percent of the total project cost

Architect: Fee is typically 5% to 20% percent of the total project cost

Carpenter: $75 to $100 per hour

Excavation and demolition: $500 to $10,000 (includes permits, site prep, equipment, etc.)

Electrician: $200 to $300 per hour

HVAC technician: $150 to $300 per hour

Plumber: $200 to $300 per hour

Roofer: $50 to $100 per hour

Factors That May Lower Addition Costs

Limit the Size

Consider if you need all of the intended space and pare down any unnecessary space. For example, decreasing the size of a living area by a third will lower the cost of the addition.

Be Flexible

Remain flexible about scheduling for possible discounts. Remember that spring is usually the prime renovation and remodeling season.

Resist Unnecessary Changes

Resist the urge to make capricious or unnecessary changes. Changes always drive up the cost of an addition; do so only when absolutely necessary.

Frequently Asked Questions

Is it cheaper to add on or build up?

Building up is typically less costly than adding on. Adding on requires expensive foundation work that building up does not. This is a blanket rule however, each situation is different – it is especially dependent on what type and size of second story addition you would like to build.

How much should I budget for an addition?

Your budget depends on what type and size addition you want. To get started, review the chart above, and try to get an idea of the size of addition you are considering. While most general contractors will not share a detailed estimate for free, you should be able to get some additional information on pricing by reaching out to a few. Our direct contact is (208) 280-9213 or

How long does it take to build an addition?

This can vary widely depending on all of factors listed above but a blanket rule is 4 to 8 months of construction time and 2 to 4 months of project development and design time.

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