How Much Does a House Extension Cost? – A Comprehensive Guide in 2023

Philip Jennings |

Embarking on a house extension project in London is an exciting endeavour, but it comes with various considerations, the most critical of which is cost. As an architectural practice with thousands of completed projects, we will explore the costs associated with different types of house extensions in London, in 2023 with a focus on single-storey side extensions, single-storey rear extensions, double-storey extensions, and front extensions.

Understanding the Costs: The Basics

Before we dive into the specifics, let’s establish some fundamental principles that govern the cost of any house extension project in London.

  • Location: First and foremost, the location of your property plays a pivotal role in determining the cost of your house extension. Central London tends to be pricier compared to the suburbs, due to variations in property values, accessibility, and construction expenses.
  • Property Type: The type of property you own can significantly impact the cost. Terraced houses are more restricted when it comes to rear garden access. Semi-detached and detached houses will offer more flexibility, but still come with unique considerations.
  • Design and Complexity: The architectural design and overall complexity of your house extension project are key cost drivers. The more intricate and unique your design, the more it’s likely to cost. On the other hand, a straightforward design will be more budget-friendly.
  • Materials and Finishes:

The choice of materials and finishes is crucial. High-end materials and luxurious finishes can elevate the cost, while more cost-effective alternatives can help you stay within a tighter budget.

  • Professional Fees: Don’t forget to account for the residential architect, structural engineer, and contractor fees. These professionals are instrumental in the success of your project and their fees can vary based on their experience and expertise, however paying over the odds for these services can unnecessarily increase construction costs before building works begin
  • Project Size: Naturally, the size of your extension directly affects the cost. A larger extension will require more materials, labour, and time, ultimately driving up the budget. It’s vital to carefully plan and budget for the specific size and requirements of your project.
  • Contingency Fund: It’s prudent to set aside a contingency fund, typically around 10% of the project cost, to be financially prepared for unexpected expenses or changes in the project.

Now that we’ve covered the fundamentals, let’s delve into the specifics of the four most common types of house extensions in London:

Single-Storey Rear Extensions

Single-storey rear extensions are a popular choice for homeowners looking to expand their living space without drastically altering the property’s layout. These extensions typically involve adding a room to the rear of your home, whether it’s a spacious kitchen or cosy living area. Many cost considerations for rear extensions align with side extensions, but there are specific elements to keep in mind:

Cost per Square Meter: The cost of a single-storey rear extension in London generally falls within the range of £1,500 to £2,500 per square meter. However, this range can vary based on factors such as materials, finishes, and the level of customisation involved. High-quality finishes and unique design elements can push costs toward the higher end of the spectrum, while a more modest design and materials selection can keep expenses in check.

Considerations for reducing the cost of a single storey rear extension include;

  • Glazing: Generous glazing for an indoor-outdoor connection can increase costs. However, it’s an investment in aesthetics and the connection with your outdoor space.
  • Removing structural walls involves the insertion of steel beams which can considerably increase construction costs. Partially maintaining load bearing walls or maintaining existing openings can greatly reduce overall construction costs

Based on our recently completed project, you can expect to pay in the region of £40,000 – £90,000 for a single storey rear extension, dependant on the factors mentioned above.

Single-Storey Side Extensions

Single-storey side extensions are an excellent choice if you have lateral space available. These extensions can be used to expand your kitchen, create a utility room, or establish a comfortable home office. It is also a good idea to consider if you are hoping to maintain the rear garden space. The cost of a single-storey side extension typically aligns with that of a rear extension, ranging from £1,500 to £2,500 per square meter.

Cost per Square Meter: Like rear extensions, the cost per square meter for single-storey side extensions generally falls between £1,500 and £2,500. However, the ultimate cost depends on design complexity, material choices, and the scope of the project.

Considerations for reducing the cost of a single storey side extension include;

  • Standardise windows and door sizes. Custom-sized windows and doors can add significant costs. Standard sizes are more readily available and can be more budget-friendly.
  • Simplify the roof design. The roof design can have a substantial impact on costs. Simple roof structures and designs are often more cost-effective compared to complex or unique shapes.

Double-Storey Extensions

For those in need of significant additional living space, double-storey extensions are an ideal solution. Whether you’re adding extra bedrooms, a spacious family area, or more, these extensions can be a game-changer for your home.

Cost per Square Meter: Double-storey extensions are often slightly more expensive due to the increased complexity and structural work required. As a result, you can expect costs to range from £2,000 to £3,500 per square meter. The specific cost will depend on various factors, including the design, materials, and finishes chosen.

One of the main cost benefits with double storey extensions is an efficient use of foundations. When you build upwards with a double-storey extension, you can often use the same foundation footprint as the ground floor. This results in more floor space without additional foundation costs.

Considerations for reducing the cost of a double storey extension include;

  • Ensuring the footprint of the first floor, matches that of the ground floor. This will ensure, additional steel supports are not required to support the upper floor, instead it can be supported on the walls of the ground floor.
  • Work with an experienced engineer. An experienced engineer will specify the exact support required for a double storey extension, without over-specifying. Over-specifying support requirements can lead to more expensive material and labour costs.

Front Extensions

Front extensions are an exciting option to consider when you want to enhance your property’s curb appeal and create a more welcoming entrance area. When considering a front extension it is important to work with a experienced residential architect as front extensions are subject to more strict planning restrictions.

Cost per Square Meter: Front extensions offer a diverse price range, starting at £1,500 per square meter. However, costs can rise depending on the complexity of the design and the materials used.


Understanding the cost of a house extension in London involves considering a multitude of factors, each of which plays a crucial role in shaping your budget. To embark on your project confidently, we recommend seeking advice from an experienced residential architect, obtaining multiple quotes, and creating a detailed budget that accounts for all the influencing factors.

Remember, while cost is a critical consideration, investing in a well-designed and functional extension can greatly enhance your living space and property value in the long run. As architects, we’re delighted to be a part of our clients journey in creating their dream home. Every project is unique, and our goal is to ensure your vision becomes a reality within a budget that aligns with your needs and aspirations.

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