What is the Cost of an Extension?

An extension offers a potential bump in your home’s value, added versatility for your home, and more room. They range from building walls and laying foundations, to plumbing, electrics, and plastering. Other features such as interior design and unique flooring have to be considered.  

All these are factors that affect the expenses of an addition. We have developed a house expansion price guide to offer the basic knowledge you need to start planning your expansion project. Included are the different kinds of expenses involved. You need to understand the key underlying expense is the size of the addition you want to be built.  

The basic cost of an extension is $1,400 to $1,800 per square meter. This price bracket reflects the variants of your location, quality of finish, the complexity of the design and the materials used.  

The essential elements of the build are covered by the price range. They include building the walls, constructing the foundation, adding a subfloor, fitting out with plasterboard, insulating the new structure, and installing a basic provision for electrics and plumbing.  

Any finishing elements are not included. For instance, kitchen installation or interior décor is not covered.  

Happy DIY Home Top Tip

  • Half the price for twice the height? Some people believe adding a second story to an expansion will cost a fraction of building a single story. Building contractors describe this as a myth. The key factor is the total floor space. If you plan on getting a lot of floor space, ensure your budget is sufficient. 

Where to Find Extension Specialists

Extension Cost Per Square Foot

1 New Extension Cost Per Square Foot
In construction by sekihan / CC BY-NC 2.0 Decide on how large you want your extension to be first, and then work out what you want to use the room for. This will give you a rough idea of your total costs, so you won’t blow your budget. 

There is an impressive range of costs associated with adding an extension onto your home, depending on how much existing space you use, the room’s intended use, and where you want to add on. Generally speaking, you’ll pay roughly $200 a square foot for your extension cost. Complex projects can increase the price per square foot while very simple extensions can lower the price. You can get a rough estimate for your extension cost based on the following: 

200-Square Feet

On average, adding a 200-foot extension will cost you around $40,000. This size extension is more than large enough to house a master bedroom, kitchen, family room, or living room. The exact price you end up paying will depend on what you want to do with your space, with living rooms costing less than a kitchen. The more elaborate your extension is, the higher and quicker the price jumps. Things like heavier door or window trim and crown molding can create higher costs. 

300-Square Feet

You’ll pay around $60,000 for a 300-square foot extension. An extension this size can comfortably accommodate a kitchen that has a breakfast nook or a family room. You could also use this size extension to create a combination laundry room and mudroom. What you do with the space will dictate your final price, as with any extension. Extensions that need plumbing work cost more than those that don’t need it, and decorative details will also increase your costs. 

400-Square Feet

On average, you’ll spend $80,000 for a 400-foot extension. If you choose to create an extension this size, it can accommodate multi-use areas. So, you could have a dining room and kitchen, or you could choose a master bathroom with a bathroom attached. When you hit this size, you could also consider a two-story addition while dedicating 200-square feet to each floor. These variables can easily cause your costs to fluctuate. If you add plumbing in a bathroom or kitchen, or if it’s a two-story addition, your costs can  jump. 

500-Square Feet

On average, you’ll pay around $100,000 for a 500-square foot extension, and you’ll get a huge range of possibilities at this size. You can expand your open floor plan by adding a single large room, or you can easily incorporate multifunctional rooms. You could have a big kitchen with a dining room or breakfast bar, or you could create dual bedrooms connected with a Jack-and-Jill bathroom. Plumbing will run up the cost of your extension at this size too. 

600-Square Feet

On average, your 600-square foot extension ranges right around $120,000, and it’s roomy enough to give you huge design potential. You could easily add a full-sized master suite that offers a large master bedroom, walk-in closets, bathroom, changing areas, and a sitting area. Another option at this size would be to create two large bedrooms with a Jack-and-Jill bathroom, a home office, or a two-story addition if you’re not afraid of higher costs. 

800-Square Feet

You’ll spend around $160,000 on average for an 800-square foot extension. You could add it on as a partial second-floor extension. So, you could add it on as a large floor above your garage, and have a connecting hallway to the main house. This size also lets you create a two-story extension with a large room on each floor. You have to factor roofing or plumbing into the equation if you choose to go this route, and it can increase your total costs significantly. 

1,000-Square Feet

Finally, a 1,000-square foot extension will usually cost around $200,000. This is a very sizable addition to any home, and it could potentially double the square footage of your existing space. An extension of this size is usually reserved for a partial or full second-story home modification. It could also be a garage addition with a living space situated above it. If you choose to go this large, ensure that you have the zoning and space available to complete it. 

Stages of an Extension Project

Extending a building is done using several stages of building broken out below. Establishing prices for each section is a challenge.  

One reason is the majority of builders do not separate expenses for elements like groundworks. Such elements are rarely done in isolation from other elements of the project. 

Many homeowners building an expansion will agree to pay their tradesmen and workers in stages. Typically after the below elements are completed. 

2 Extension Project Stages
House Construction by Mary Molinaro / CC BY-NC-ND 2.0 Part of the stages of building an extension

Planning Permission Costs and Design

Some people choose to use the services of an architectural technician or architect to draw up plans. Others opt to hire a contractor to design and build their extension. The rest choose to go the D-I-Y way.  

Architectural drawings will set you back between $650 – $1,300 depending on the complexity and scale of work.  

Permission for an addition by the relevant state authority is not always necessary for an expansion project. In some cases, you may need to add about $250 depending on your circumstances and the area.  

Ground Works

Preparing the site and digging the foundation is crucial to ensure the extension is built with structural integrity and safety. Development expenses for this phase are hard to separate in case complications arise.  

Examples of complications include dealing with tree roots or moving drains. Such complications increase the expenses by slowing down the construction process. 

The Shell

It is the basic structure of the building. The shell is built with either timber, block, or brick frame. The roof is built, and interior features like a subfloor and plasterboard are added to make sure the structure is watertight.  

Charges here typically relate to the size of the build. Currently, timber, blockwork, and brick frame developments are at a similar price point.  

Each has various drawbacks and advantages in certain situations. However, the main building material choice is not a major concern for an average expansion job.  


This is where elements such as switches, electrical outlets, doors, and HVAC systems are installed in the extension. The cost of all these features depends on whether you want to buy high-end, fancy, or simple materials.  

For example, you may want to buy sliding doors. These are substantially more expensive than simple French doors.


There is no limit on the cost of finishing a development. Every feature has a widely different expense.  

For instance, imported marble tiles are more expensive than local porcelain tiles. Expenses increase significantly if you plan on adding a new bathroom or kitchen to the extended space.  

As a benchmark, a new bathroom could cost between $ 3,500 and $13,000. A new kitchen goes for between $ 8,000 – $30,000. 

Different Types of Extensions and Their Price 

A price per meter is the best way to get an estimate or actual cost of any development. The type of expansion determines the total amount of money used.  

Cost of Rear Extension vs Side Extension 

Floor space is the main calculation for pricing any extension. It will not matter whether it is at the side or rear of a property as this will not largely impact the total cost of the build.  

Other factors will affect the pricing depending on the design. For instance, you will need to consider how it will integrate with the original building. You also need to consider the structural implications. 

Things like how many doors and windows need to be considered. Features such as a new bathroom or kitchen, and how easy it is to access the structure determine the overall cost. 

3 Different Types of Extensions
New House by squiddles / CC BY-NC 2.0 An example of a side extension in progress.

Single vs Double Story Extension Costs

Many sources claim a single story will be priced at twice or more of the amount used on the second story. For example, if a single-story structure will set you back $30,000, the first floor will be an additional $15,000 for a total of $45,000.  

However, our research has found this to be false. The majority of builders calculate the cost of the job based on total square meters. This means a two-story structure with a total floor space of 40msq will be $60,000.  

It may seem intuitive that a ground floor requires more labor due to the groundwork and foundations. The first floor has additional expenses involved not applicable to a single-story project.  

For starters, you need to change the roof, increase structural integrity with the main building, work at a height, and use scaffolding. All these factors end up balancing out the cost. 

Cost of Timber vs Masonry Frame  

Masonry or “block and brick” construction is the most common way of building a home in most parts of the world. Needless to say, timber projects are gaining popularity, especially in the United States.  

Timber frames use wood to build core panels assembled on site. Masonry construction uses blocks or other forms of exterior cladding around it to build the structural element. 

Each has drawbacks and benefits in terms of structural benefits and speed of construction. In terms of charges when you build an extension, the two methods are fairly equivalent.   

Attic Extension Cost

It typically costs between $40,000 and $50,000 to add an attic extension. However, this is typically an attic conversion versus adding on a whole new extension. It’s a larger project, and you have to ensure that you have an egress like stairs and that it works with your lower ceilings. So, you might end up with a much smaller area for the price. 

Bedroom Extension Cost 

Your extension cost for a bedroom ranges from $35,000 to $100,000 on average. This extension can be slightly smaller, but they have a need for features that other rooms don’t. One example would be closets. Necessary lighting and storage also raises your costs to around $57,000. 

Family Room Extension Cost

To add a family room to your home, you’ll pay between $40,000 and $60,000. This room comes in a large range of sizes and shapes, and you can finish it in several ways. This room is usually more informal than others in the house, and they typically have entertainment areas, storage, and enough space to comfortably spread out. 

Kitchen and Bathroom Extension Costs

Planning to install a bathroom or kitchen in the new space will impact the overall price of a structure. The price of new bathrooms and kitchens vary widely. The price is determined by the quality of the products and your requirements. 

To add a full bathroom, you’ll typically space out 48-square feet that will drive your extension costs between $20,000 and $60,000. Even though the bathroom is usually a small space, it needs plumbing. This raises your costs. An extension with a bathroom has to include everything from the toilet and sink to the shower, along with any accessories and tile

An extension with a kitchen starts at $50,000 and goes up to $70,000. Kitchen extensions typically have added prices for plumbing and cabinetry. So, while the average kitchen size is between 100 and 200-square feet, the cost is significantly higher than other areas. The price includes cabinets, flooring, appliances, counters, and finishing and framing. 

4 Kitchen Extension Project
Kitchen Cabinets Going In by plien / CC BY-NC-ND 2.0 A kitchen or bathroom are two of the more expensive extensions to consider, even if you want smaller rooms. They have more features and needs that can drive your extension costs up. 

Garden Room Extension Costs

Garden room style extensions are usually design-focused. Typically they are a single story with an emphasis on sliding doors or large windows allowing easy access from the house.  

Several factors impact the final cost of a garden room extension project. Desired features like the type of doors and materials you use will have an impact on the price.  

Laundry Room Extension Cost

To add an extension for your laundry room, you’ll typically pay between $5,000 and $8,000. The typical size for this room is around 35-square feet, and this makes this extension fall into a bump category rather than a full extension. Any laundry room should be waterproof, and they usually have storage spaces and cabinets installed to maximize the space. 

Living Room Extension Cost

Your living room can run between $40,000 and $60,000 for an extension cost. This is usually a slightly smaller room, but it’s more formal than a family room. So, you may have less space but you’ll use slightly higher-end materials like marble or exotic hardwood floors, nicer moldings, and nicer features. All of these items can drive your costs up. 

Home Theater Extension Cost

Adding a home theater to your existing space costs between $20,000 and $70,000. It adds entertainment and fun to your home, and you can add it to a basement, existing space, or you can build it into your home if you don’t have room. Because home theaters require a special focus on equipment and acoustic materials, they can have a huge cost range associated with them

Mudroom Extension Cost

Your mudroom will cost between $10,000 and $14,000 when all is said and done. This extension usually falls around 50-square feet. So, when you include the organizational materials needed and cubbies, the costs usually don’t exceed $12,000. Typically, you’ll have storage, cabinets, and durable flooring factored into your final extension cost. 

Sunroom Extension Cost

A sunroom also has a broad cost range that starts at $35,000 and can easily exceed $55,000 or more. It can take on a range of forms and styles, so this contributes to your extension cost. Sunrooms have full walls, screens, or windows for one, and the final design will depend on your goal. On average, you’ll pay out $45,000 for a three-season sunroom.

Room Extension Cost by Size

The size of the room you want to build with your extension will also impact your costs. Smaller rooms are naturally less expensive when it comes to pure square footage. So, you can create several smaller rooms for the price of a larger room, depending on what you want to use it for. The average room extension cost by size is as follows: 

  • 6-foot by 8-foot: $9,500
  • 8-foot by 10-foot: $16,000
  • 8-foot by 12-foot: $19,000
  • 10-foot by 10-foot: $20,000
  • 10-foot by 12-foot: $24,000
  • 10-foot by 15-foot: $30,000
  • 10-foot by 20-foot: $40,000
  • 12-foot by 12-foot: $29,000
  • 12-foot by 16-foot: $38,000
  • 15-foot by 20-foot: $60,000
  • 16-foot by 16-foot: $51,000
  • 20-foot by 20-foot: $80,000
  • 20-foot by 40-foot: $160,000

Extension Cost According to the Type

5 Finishing an Outdoor Type of Extension
164_6413 by chaim zvi / CC BY-NC 2.0 The type of extension you want will factor into the costs as well because some are much more involved and complex than others, and this can push your costs up relatively quickly. 

There are several ways to add an extension to your home to help extend how much space you have. You don’t have to add a full extension to get these benefits either, especially if you’re going to a smaller room over a large addition. The various extension costs by type include: 


An extension cost for a balcony ranges from $4,000 to $9,000. It’s a small deck that gets suspended off the side of your home, and it works well to update your home’s exterior. If you don’t have room for a full porch or deck but you want somewhere to sit outside and relax, a balcony is a nice option. They come in a huge range of styles and sizes to complete any design aesthetic. 

Bump Out

A bump out is a smaller extension that goes into an existing room, and it has a price tag of $4,000 to $9,000. You’ll get more square footage at a lower cost. You’re more likely to add this type of extension on your first floor, but you can also add them to a second story. This is a good option if you’re trying to add storage space to an existing room or put in a laundry room. 


The extension cost for this project ranges from $15,000 to $25,000. It’s a nice way to add a room to your home’s second story by cantilevering it out over the first story. It’s not a large space, but the costs go up due to a need for more support. You can use posts for support, like you would a deck. 


A dormer is a type of roof extension that includes a window, and your extension cost with this project ranges from $4,000 to $9,000. It’s one way to add extra space to an attic or low-ceilinged room. The domer works like a bay window as it can give you a window seat for more storage. If you have higher ceilings, it can give you more standing room. It’s a nice extension for the exterior since it’ll change your home’s profile and make it look nicer to potential buyers. 


A garage extension will cost between $22,000 and $43,000. Having an attached garage can increase your home’s value, and it improves how functional it is. It makes your vehicles more secure and protected from the heat, storms, or other elements. There is also a huge potential for storage opportunities with garages too.


On average, your greenhouse will start at $5,000 and go up to $20,000. If you do a lot of gardening, adding a greenhouse as part of an extension project is both practical and functional. You can add it directly to your existing home using a lean-to structure, or you can build it 100% separate from your home. They come in a large range of sizes and shapes, and each style comes with different benefits, drawbacks, and costs. 

In-Law Suite

Adding an in-law suite onto your home costs between $60,000 and $100,000. The final cost will depend on the size. This is a fully-fitted apartment or building that is attached to your home. They often have bathrooms and kitchens in them, and this can drive their costs up. This is a great way to accomplish multi-generational living in a single home. They do require a separate entrance from the main home, and they need all the traditional features that a stand-alone apartment would. 

Modular Room

A modular room extension will cost between $20,000 and $30,000. Modular building has a lower price tag than traditional because the room comes factory-built, delivered, and attached to your existing home. The price per square foot is also lower, and it has a fast timeline. They can also get made in any style, configuration, or size to match your home in a seamless manner. 


A porch extension ranges from $12,000 to $30,000. Having a porch on your home boosts the curb appeal and value, and it also gives you outdoor living space. They come in a range of styles and sizes. Also, porches give your home ample space for entertaining or recreation, and they improve your home’s general appearance. 

Extension Cost – Frequently Asked Questions

We understand that it’s very common to have questions when you’re trying to figure out whether or not you should justify an extension cost to your home, so we’ve rounded up the most frequently asked questions and laid them out for you below. 

1. Do you need a permit to build an extension?

Yes. You’ll need the proper permits to build an extension, and you should factor this into your extension cost. You should contact your local city or town hall a few weeks before you begin your project to ensure you have the time to file for the permit and get it back. 

2. How do you keep your extension costs low?

One of the biggest things you can do to keep your extension costs down is to keep the room simple. If it needs plumbing, the costs will go up. A host of cabinets or storage options can also increase the price, as can more high-end details. 

3. How long does an extension take to finish?

This will depend on how involved the project is, the final use of the room, and the size. At the very minimum, plan on it taking two or three months. This can easily go up to six to eight months or more, depending on how complex it is. 

Bottom Line

Never embark on a building extension project unless you are fully aware of the cost and job involved. Start by setting a budget and getting quotations from different contractors.  

Allow yourself a contingency. This will ensure you cover any unforeseen expenses due to issues or delays with materials.  

You can save money when building an expansion. For example, you can decide to do some of the jobs such as interior decoration and painting. Doing as much as you can on your own is the best way to reduce the price of building an extension.

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