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

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. 

extension cost
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 you $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. 

2 cost of an extension
An example of a finished side extension.

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.   

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. 

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.  

3 extension cost estimate
Exterior of a garden room extension

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.