How Much Does a Home Addition Cost?

Of all the possible remodeling projects done to a home, the constructing of an addition has by far the most variable scenarios to determine its cost. Thus, attempting to do research online or by reading home remodeling magazines to determine an idea of cost is almost impossible. Besides the obvious variations such as the size of the addition, the number of rooms, types of rooms, and the number of stories, there are other variations, some of which you have no control of. They are (1) Surrounding Circumstances (externally & internally), (2) Design/Materials, and (3) Upgrades to Comply with Building Codes & Zoning Requirements. These variations differ from one home to another, thus constructing the exact same size addition to different homes can still be a possible 20% to 25% difference in the final cost.

When homeowners begin to think about building an addition to their home, one of the first questions they ask is, "how much will this cost?" Unfortunately, there's no quick answer to this question as explained above. There are many websites and home remodeling magazines that cite a range of cost-per-square-foot for home additions. However, this range is so flexible that it's truly not helpful knowledge for your guidance to be assured you could afford to have one built or not. Also, not mentioned, nor included online are the three variables mentioned above.


My company, Arnone Building & Remodeling, spanning six decades in the home improvement industry, builder/designer of over 500 additions, ranging from $ 5,000.00 to $ 367,000.00, is advising that it's difficult to learn of cost by researching topics of this subject online or by reading home remodeling magazines.

Why You Can't Estimate Cost by Looking Online (Read Examples 1-4)

I've never calculated the cost of any addition by solely using a cost-per-square-foot factor, which is regularly mentioned when researching online. I calculate the cost of every addition on an individual basis: (1) Square footage of Addition & Lineal footage of exterior walls (different from just square footage), (2) Surrounding Circumstances, (3) Design/Materials, and (4) Upgrades to Comply with Building Code & Zoning Requirements.

I'll enlighten you on all these variable scenarios for your better understanding, and I'll illustrate the flaws and omissions regarding online research.

Based on Square footage of Addition (Example 1A)

Usually, in a given scenario, the square footage of the proposed addition is multiplied by a suggested per-square-foot range of cost.

Let me explain why this information is incorrect/flawed.

For example, a 12' x 24' addition (288 sq. ft.) is not going to cost twice as much as an addition half its size (12' x 12', 144 sq. ft.). The larger size addition (12' x 24'), although overall more expensive, is going to average less per-square-foot to construct. I'll explain in the comparison of these two sizes mentioned.

Ground-level additions have three exterior walls. The fourth wall is actually the butting wall of the existing house. The total linear footage of the combined three new exterior walls of a 12′ x'12' addition is 36'. These 36 linear feet of construction consist of excavating, forming and pouring foundation walls, exterior wall framing and sheathing, windows and doors, exterior wall siding, interior wall insulation, drywall and interior baseboard molding and electrical wall outlets. That being said, an addition 12' x 24' (twice the square footage) doesn't have twice the linear footage of these various types of material and labor cost to install. The linear footage of the exterior walls of a 12' x 24' addition is 48', only 33% more than 36', not twice as much.

Based on Square Footage/Linear Feet of Exterior Walls (Example 1B)

  • Windows and Doors - A 12' x 12' addition, for example, may have one door and one window on one wall and two windows on each of the remaining two walls-in all, a total of one door and five windows. In comparison to a 12' x 24' addition (twice the square footage), still one door and one window on one of the 12' walls and two windows on the remaining 12' exterior wall, with the difference being four windows on the 24' wall. In all, a total of one door and seven windows. The comparison difference in the addition twice the square footage is just two additional windows and no additional doors-as opposed to 10 windows and two doors (double the amount of a smaller size addition) and the flawed theory that an addition twice the square footage is twice the cost.
  • Square Footage of Exterior Walls (Based on using the same number of windows and one door ) - On the 12' x 12' addition, the square footage coverage of all three exterior walls (minus a door and four window openings) is 201 square feet. Meanwhile, for a 12' x 24' addition, the square footage coverage of all exterior walls (minus a door and seven window openings) is 273 square feet. Thus, doubling the size of the addition, your materials and labor costs will only increase by 35%, not 100 % more (double).
This 35% increase in materials and labor translates to (1) Exterior Wall Framing & Sheathing, (2) Exterior Wall Insulation, (3) Exterior Siding, (4) Drywall (hang and tape), (5) Electrical Wall Outlets, (6) Base Molding and Interior Trim on Windows and Doors, and (7) Also, the Excavating, Forming, and Pouring the Foundation Footings and Three Foundation Walls.

There are some scenarios in which there will be double the amount of material and labor on the 12' x 24' addition (twice the size). They are (1) Ceiling insulation, (2) Floor Insulation, (3) Roof & Floor Framing, (4) Finish Flooring, (5) Drywall on Ceiling, and (6) Concrete Slab Floor in the Addition, Basement, or Crawl Space.

Based on "Surrounding Circumstances" Externally (Example 2A)

These possible scenarios aren't usually mentioned/included and are often overlooked when you research to understand the cost of your addition online. These examples below (translating to an additional cost) have to do with surrounding exterior areas and vary with each house. Thus, another example of how the same size addition placed on two different homes can vary in cost.

  • Is there a deck, concrete patio, porch, concrete steps, or sidewalk that must be demolished and re-built?
  • Is there a sewer line within the perimeter of the proposed addition that must be rerouted?
  • Is there an underground drainage pipe along the perimeter of the house foundation that now has to be removed in the area where you are putting the addition, which will then need to be re-routed and re-connected along the perimeter of the new addition?
  • Is there an air conditioning unit, electric meter box, water faucet, cable box, oil fill, or gas meter that will need to be removed and relocated?
  • Are there trees or shrubbery that must be cut down or relocated?
  • Can heavy equipment such as a dump truck, backhoe, or cement truck access the area where the proposed addition will be installed? If there is no access for a cement truck, then concrete will have to be pumped in on each of the three separate pours (footings , walls & slab floor in basement or crawl space ) (from driveway or street).
  • What will be the height of the foundation walls of the addition above grade? This is a variable based on the terrain of one's existing property. The same size addition on one property could need a 2' high foundation wall and on another property need to be 5' high.
  • Another expense in the construction of an addition to consider in your budget is lawn repair due to heavy equipment such as a backhoe, dump truck, concrete truck, and excavating.

Based on "Surrounding Circumstances" Internally (Example 2B)

Electrical Upgrades
All additions require electricity, which places a burden on the existing electrical system. If the home's electrical panel box has adequate space for the additional required circuits, then no electrical upgrade/cost is required. If not, then an additional cost will be required to either install a sub-panel or remove the entire electrical panel box and install a new 200 amp upgraded electrical panel box (this varies from home to home).

Heating System & Air Conditioning
All additions require heat and, in some cases, air conditioning. The existing home's furnace or boiler and air conditioner unit are sized slightly more than the present cubic footage of the existing house. If the home substantially grows in size, the furnace or boiler and air conditioner may not be able to keep up with the extra demand. During the estimating stage, the furnace or boiler and air conditioning unit must be inspected to determine whether or not it is big enough to heat or cool the updated cubic footage effectively. If not, additional cost (this varies from home to home).This is the text area for this paragraph. To change it, simply click here and start typing.

Connection to Each Mechanical System
Usually, the connection to the boiler, furnace, waste pipe, or electrical panel occurs in the basement. Is the route from the house equipment to the addition clear, or are there obstacles? Is the basement finished as opposed to unfinished? Is the distance between the addition and the equipment far or close? Another example when including a bathroom in addition, is the connection route of the drain pipe from the proposed addition to the basement connection in the house close or far away? Is there adequate pitch for drainage or will a pump be required?

Design and Materials (Example 3)

A major part of the cost of an addition is mimicking the original design of the house. If the home is made from expensive materials, has a design that is difficult to replicate, or requires extra supplies, this could inflate the cost significantly.

Present house examples to mimic on addition, include:

  • Is the exterior siding on the house vinyl siding or either cedar clapboards or wood cedar shingles? The wood choices are three times as expensive, material, and labor wise, to install. Plus, you'll have an additional expense to either stain or paint the wood siding.
  • Is the pitch of the house roof steep or walkable? Material and labor to install are more costly if steep.
  • Does your house have roof overhangs over the gable walls?
  • What is the depth of roof overhangs of house (in inches where the gutters are located ) , can vary from 6" to 24".
  • What is the roof style of your house? Gable or Hip? A hip style roof is more costly to construct (material & labor wise).
  • What is the height of your house walls? They can range from 7' 6" to 9' 0" high. This reflects the amount of wall framing/sheathing, exterior siding, wall insulation, and drywall needed.

Your selections based on your personal taste/budget:
  • Installation of stain-grade interior trim versus paint-grade.
  • Installation of interior solid pine doors versus hollow core doors.
  • Oak flooring/tile flooring or less expensive vinyl flooring or carpet.
  • Cathedral ceilings and skylights.
  • Attached deck? If so, pressure-treated wood or composite decking.
  • Interior entrance into the new addition. As inexpensive as removing the original exterior door and trimming this existing opening or taking down the entire wall (bearing) between the house and new addition, thus installing a structural support header and having to re-route electrical wires and maybe plumbing in this wall.
  • Addition, either a crawl space or a full-height basement.
  • Interior lighting, single-center ceiling fixture, or multiple hanging/recessed lighting.

Upgrades to Comply with Building Codes & Zoning Requirements (Example 4)

Cities / Towns Vary
Depending on where you live, in some areas of the state, it's required to excavate and bury plastic chambers in the ground, acting as a dry well to collect water drainage coming off the new addition roof. The logic being, the structure of the addition is now occupying ground space that previously absorbed rainwater in the ground.

Septic System or City/Town Sewer Connection
If your addition includes a bedroom and you have a septic system opposed to a city/town sewer connection, then there's the possibility of having to enlarge your septic system to accommodate additional bedrooms or prove you have an adequate reserved area in case you later have to enlarge the septic system. Contrary to having a sewer connection, you can add as many bedrooms as you want without any restrictions/costs. Another example of how the same size addition to one home can vary in price at another home.

Applying for a Variance?
Additional cost if you do with no guarantee you will be approved. Basically, infringing to build an addition beyond the set-backs of adjoining properties including street.

Existing Conditions
There may be existing pre-existing violations noticed that would be required to be corrected that were either initially noticed by the contractor during the bidding stage or during the inspections by town inspectors of the addition. Some examples (but not limited to) are, (1) Built structures without receiving a permit. Usually, if obvious violations are noticed then, no permit was issued, (2) The door between the house and garage is not fire-rated, and (3) Absence of smoke detectors.

Get a Professional Estimate from Arnone Building and Remodeling

Instead of struggling to figure out the approximate cost of the addition you're considering to build by going online and reading the wide range of costs based on square footage along with now knowing all the omissions and variables I mentioned earlier, call Arnone Building & Remodeling.

At Arnone Building and Remodeling Inc., we begin this process by scheduling an in-depth phone conversation. We'll discuss the specifics of your current home as well as what kind of addition you're interested in. We will also take a look at your property via Google Earth to determine the space and terrain we will be working with. This information is typically enough for us to provide an approximate quote for your better understanding. When you are ready to move forward, we will then schedule an on-site, in-person consultation to discuss the project in greater detail.

At Arnone, we believe that in-person discussions are at the heart of good bids and estimates. Only with extensive research and communication can we truly determine the real cost of the job

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