Want More Space? A Comprehensive Guide to Room Addition Costs

You love your place, but honestly, you could use a little more space. Perhaps you have a new baby on the way, the kids are grown and need their own room, or you have a family member moving in to stay awhile. If you stretched for a new primary suite, or created another spare bedroom, you could transform your house into a forever home. So around how much would a room addition cost?

According to HomeAdvisor, homeowners spent on average $11,520 to $28,000 on 12 x 12 room additions and $32,000 to $80,000 on 20 x 20 room additions. That’s a helpful start, but those are pretty big windows.

“You can’t just say that this size addition is going to be X amount of dollars per square foot,” says Jim Pitzen, a licensed architect and Design Consultant for the S.J. Janis Company with over 20 years experience building residential additions. “Site conditions, details, what we’re touching, what we’re not touching — that all comes into play with the overall cost of that addition.”

To help you craft a custom estimate, we’ll break down how these factors influence the cost of a room addition. Our comprehensive guide to room addition costs covers:

  • Room addition cost overview
  • Room addition costs in your market
  • Cost per square foot by room size
  • Project costs by placement
  • Cost breakdown by component
  • Common challenges and related costs
A notepad used to figure out the cost of a room addition.
Source: (Kelly Sikkema / Unsplash)

Room addition cost overview

As Pitzen notes, the cost of a room addition varies dramatically from project to project. For some ballpark figures, let’s compare averages between the web’s leading sources on remodeling costs:

Addition Size HomeAdvisor HomeGuide
Average cost (all sizes) $21,075 – $69,887 $22,500 – $74,000
Average cost per sq. ft. $80 – $300 $86 – $208
12 x 12 (144 sq. ft. ) $11,520 – $28,800 $12,400 – $22,300
15 x 15 (225 sq. ft. ) N/A $21,000 – $52,000
20 x 20 (400 sq. ft.) $32,000 – $80,000 $34,400 – $83,200
600 sq. ft.  $48,000 – $120,000 $51,600 – $124,800
800 sq. ft.  N/A $68,800 – $166,400
1,000 sq. ft.  $80,000 – $200,000 $86,000 – $208,000

Methodology: HomeAdvisor collected survey responses from 4,120 members who recently completed a home addition. HomeGuide tracks millions of estimates from local companies to create reports based on project averages; they do not specify how many projects were compared for these home addition averages.

A bedroom that is a room addition.
Source: (S.J. Janis Company, Inc.)

Estimate room addition costs in your market

Next, narrow in on a project estimate based on material and labor costs specific to your market. Both HomeAdvisor and HomeGuide offer project cost calculators that factor in your location, addition size, and function (i.e. kitchen, bathroom, bedroom, etc.). But if you’re hungry for real, hyper-local intel, you should meet with a real estate agent experienced in additions and remodels.

“What I specialize in is the impact of a renovation on the value of a home,” shares Allison Harris, a top real estate agent in Savannah, GA with over 217 transactions under her belt. “I make lengthy spreadsheets and cost-benefit analysis on these things to figure out exactly what kind of value you’re adding … I could even spitball some numbers and help [homeowners] figure out what the renovation is gonna cost because I do a lot of renovating with clients.”

Beyond providing realistic project figures, Harris connects homeowners to reputable contractors and architects, facilitating their access to the best quotes possible.

The larger the room addition, the cheaper the cost per square foot

If you’re dreaming of a sizable addition, here’s some good news for you: large home additions don’t always cost significantly more than smaller plans. Pitzen elaborates:

“You can’t say something 200 square feet is going to cost this much and something 400 square feet is going to cost twice as much — it doesn’t work that way. If you take a project and try to reduce it by half, the square footage, you’re not reducing it by half in cost. Typically the smaller the addition, the higher the square footage cost could be because there’s efficiency in scale and scope.”

Homeowners expend a large portion of their budget on the first stages of the build such as drawings, permits, wall removal, excavation, and foundation (if necessary). According to Bankrate, doubling the size of an addition will only increase the price by one-third.

Additionally, Pitzen points out that all professionals involved in the project will typically require a base payment for their services. For instance, an electrician may charge the same amount for wiring a 12 x 12 addition as they would a 15 x 15.

A deck that was added to a house.
Source: (S.J. Janis Company, Inc.)

Project costs vary depending on placement

Where you build your room addition greatly influences the total project cost as each placement. As you peruse these cost rundowns, bear in mind that the cheapest placement option is not always best. A logically placed room may cost more to build, but ultimately may yield more return when it’s time to sell.

First floor home addition

Average cost: $80 – $200 per square foot

Placement related costs: Excavation ($50 to $200 per cubic yard of dirt removed), foundation ($4 – $7 per square foot), and wall removal ($1,200 – $10,000)

Placement related savings: No full staircase required (save $920 – $2,880)

Expert advice: A first floor home addition is an ideal spot for a new main suite. Harris shares:

“People want the [primary bedroom] on the main, hands down. You can see that over and over again in new construction neighborhoods. Really the only people who want [the primary bedroom upstairs] are people with children under five. Once those children hit five years old it seems to be less of a priority.”

Second-floor room addition

Average cost: $100 – $300 per square foot

Placement related costs: Staircase addition ($920 – $2,880)

Placement related savings: No excavation (save $50 to $200 per cubic yard of dirt removed) or foundation (save $4 – $7 per square foot) needed.

Best usage: Purpose a second-floor room addition as an additional bedroom and bathroom for optimal return on investment. “You’re gonna see the biggest increase if you have a 3-1 and you turn it into a 4-2 or 3-2 — that’s gonna be your highest return. Going from one toilet to two is usually game changer, especially in historic neighborhoods,” Harris comments.

Above the garage room addition

Average cost: $115 per square foot

Placement related costs: Staircase addition ($920 – $2,880)

Placement related savings: No excavation (save $50 to $200 per cubic yard of dirt removed) or foundation (save $4 – $7 per square foot) required.

Best usage: Purpose an above garage addition as a bonus room or guest suite rather than a bedroom for a full-time house member (think garage door noises).

Bump-out addition

Average cost: $5,000 – $30,000 total project cost

Placement related costs: Minimal costs relating to wall removal ($1,200 – $10,000)

Placement related savings: Extending from the main building, bump-outs typically do not require new electrical, plumbing, or foundation. They also require minimal roofing, saving you 10 – 15% the cost of a full-size room addition.

Best usage: Bump-outs are a cost-effective solution for extending kitchens, living areas, and bedroom closets. Generally, contractors can expand your home 3 feet deep and as long as 10 to 12 feet wide without additional foundation work, increasing your square footage with minimal disturbance.

Sunroom addition

Average cost: $20,000 – $70,000 for a 200 square foot sunroom

Placement related costs: Foundation $4 – $7 per square foot

Placement related savings: Simpler than true room addition, sunrooms generally require fewer subcontractors and materials.

Best usage: The cost of a sunroom increases substantially when it is designed for four-season use (as much as 20%!). These sunrooms require increased insulation, higher-grade materials, and heating to keep them toasty in the winter months. If you’re uncertain which route to take, consult your real estate agent on which design is more desirable in your market.

A sunroom that was added to a house.
Source: (S.J. Janis Company, Inc.)

Cost breakdown by component

For a more detailed picture, let’s dive into the averages of the main project components:

  • Architectural fees: Architectural fees account for 10 – 17% of the total building budget. For example, if your 12 x 12 addition costs $20,000, your architectural fees will run between $2,000 to $3,400.
  • Excavation: Excavation includes digging holes, grading land, and leveling to prepare the site for foundation; on average this costs $1,200 – $4,110.
  • Foundation: For first floor home additions requiring foundation, tack on $4,400 – $12,700 per 100 square feet.
  • Plumbing: HomeAdvisor estimates that new bathroom plumbing runs between $1,500 – $4,000.
  • Electrical: Electrical work for a new room costs $2 to $4 per square foot according to HomeGuide; HomeAdvisor pins their average at $320 per 100 square feet.
  • Framing: According to HomeGuide, homeowners spend an average of $4,285 on framing for their home addition.
  • Drywall: The cost to install drywall on the floor and ceiling ranges from $1 to $3 per square foot, plus $2 per square foot for insulation.
  • Roofing: Roof framing and materials make up for a hefty chunk of home addition costs, averaging between $12,000 – $14,900 per 100 square feet.
  • Flooring: Depending on your material, the cost of flooring and installation costs averages as follows:
  • Cushion: No remodeling project is safe from unexpected costs. Harris recommends budgeting at least 10% of the total project cost for unlucky surprises.
An office that was a room addition.
Source: (S.J. Janis Company, Inc.)

Common challenges and related costs

Every room addition faces its own set of unique challenges and respective costs. Pitzen emphasizes the importance of working with a contractor or architect experienced in designing home additions who can anticipate these obstacles before construction begins:

“You want to avoid being a reality TV show. You don’t want that drama of opening up a wall and saying, ‘oh no, look what we found,’ and then we break to a commercial break and we come back and charge the homeowner another $20,000. We want to do our due diligence upfront and that’s where we are a design-build firm. We’ll put at least three months of time into just the design and prep work before we start construction.”

You’ll pay a design fee for this planning phase, but ultimately save in the grand scheme of the project thanks to increased efficiency. Here are a few common hurdles you may run into with your addition:

Termite damage

Homes with termite damage require fresh support beams to help bear the weight of an addition.  Expect to pay an extra $470 – $1,750 to replace damaged sections of wood or to add supplementary wood support. If you suspect your home doubles as a termites squat, you can conduct a professional termite inspection for an average price of $50 to $150. Termite removal costs between $220 and $906, though may run as high as $1,200 to $2,500 for full tenting.

Modifications not built to code

Pitzen refers to these pesky alterations as the “homeowner’s special,” sharing that modifications not built to code are particularly prevalent in older farmhouses which owners have mended DIY-style over the decades. Once discovered, you’ll need to bring this work up to code and pay for a retroactive permit if necessary. Cost to repair these issues vary based on the type of construction and extent of the mess. To give you an example, replacing shoddy electrical work costs an average of $6 to $8 per foot plus $40 to $100 an hour for labor.

Existing systems

Your architect may suggest shifting your room addition’s site based on the location of existing mechanical systems. Pitzen shares that moving gas and electrical lines can be moved fairly easily; moving a septic system, on the other hand, is a whole different story. A major undertaking, moving a septic system requires excavation, heavy equipment, specialized labor from a septic system maintenance company. The process is so complicated that septic tank professionals often recommend abandoning the existing system and implanting an entirely new system instead.

The average cost to move or extend mechanical systems:

A scale used to represent the cost of a room addition.
Source: (Fidel Fernando / Unsplash)

Weigh the cost of a room addition with the reward

Looking at the numbers, a room addition will set you back a pretty penny — so how can you decide if your efforts will be worth it?

“I think that anytime you do a major home improvement project, you have to look at it from two different lenses: the cost-benefit, meaning how much it’s going to cost and how much it’s going to increase the value of your home, but then also how much you’re gonna enjoy that space,” shares Harris.

For some objective expertise, reach out to a real estate agent who can help you crunch your project’s return on investment. Equipped with a market-specific ROI estimate, you can determine whether it’s smarter to renovate or to sell and upgrade to a more spacious home.

Header Image Source: (S.J. Janis Company, Inc.)