Extension cost 2024

Why consider it?

So, how much can you expect to invest in a house extension? Generally, extending your home is more cost-effective than moving to a larger property—especially when you factor in expenses like legal fees, stamp duty, and other moving costs. Plus, a house extension can significantly increase your property’s value, potentially adding anywhere from 10% to 20% to your home's worth. If you love your current location, want to avoid the hassle of relocating, or are looking to boost your home's value, keep reading! For inspiration on extensions, see what others have accomplished.

What’s the cost of building an extension? It can range from £8,500 to £20,000 for a garage conversion, while a basement extension might set you back £100,000 to £160,000, with some projects exceeding these amounts. There are countless options available, whether you’re considering building upwards, downwards, to the side, or behind your property. To get a better idea of the cost per square metre for your extension, start by using our handy chart as a baseline for this . As you contemplate moving forward, consider the value it will add to your home and be mindful of any potential extra expenses if the project runs over schedule.
 

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Types of Home Extensions
 

There are various ways to extend your home. This guide covers some of the most common types, including:
 

• Single Storey Extensions

• Multi-Storey Extensions

• Semi-Detached Extensions

• Kitchen Extensions

• Bathroom Extensions

• Garage Conversions

• Loft Conversions

• Basement Extensions
   

Factors Affecting Average House Extension Costs
 

Constructing home extensions is a complex undertaking. From the structural elements (foundations, walls, roof) to ensuring the space is liveable (electrics, plumbing, plastering), there are numerous factors to consider. The house extension prices outlined in this guide are based on the following assumptions:
 

• The costs reflect average market prices, specifications, and standard construction methods.

• Prices are for extensions on typical double-storey houses.

• No consideration is given for specialist equipment.

• The estimates include average electrical and plumbing costs.

• Unless specified, the prices are for average quality extensions.

• Refurbishments to the existing area are not included (these will incur additional costs).

• The costs should not be used pro-rata for different areas; smaller areas may result in higher costs.
   

With these factors in mind, here are some key elements that can influence home extension costs:
 

• Size of Extension: The primary expense of an extension typically comes from the build cost (materials and labor). Smaller, simpler extensions generally cost less than more ambitious projects.

• Ease of Access: Builders require easy access to the site for tools and machinery. This guide assumes a good level of access.

• Quality: The quality of extensions varies. Depending on the materials chosen, the complexity of the build, and the contractor's expertise, costs may fluctuate above or below market rates.

• Planning Permission: According to 2019 HomeOwners Survey, 27% of homeowners cited planning permission issues as a significant barrier to home improvement projects. Fortunately, many extensions can be completed without planning permission, as they fall under permitted development rights. For further information, visit the official Government site.

• Architects: Hiring an architect can streamline the extension process. They can design a space tailored to your needs (potentially saving you from costly adjustments later), secure accurate quotes, and assist with the planning permission process. Architects typically charge either a percentage of the total build cost or a flat fee.

• Finishings and Fittings: From stunning lighting to custom flooring and joinery, the aesthetic choices for your new room should be factored into the overall cost of your extension. This guide primarily focuses on simple designs, painting, and lighting.

• Groundworks: This critical task ensures the structural integrity of your extension. Additional costs may arise depending on soil type (some require specific construction methods), the need to relocate drains, or if tree roots must be removed. This guide assumes normal foundation depths with no complications.

• Insurance: Don’t forget to inform your home insurance provider about any planned construction work before it begins. While this may increase your premium, it's essential to ensure coverage during the construction phase and for your new extension once completed.

• Clearing the Space: Before any work can start, you may need to clear the area of trees, shrubs, and other obstacles. This preliminary work can add to costs, especially if contaminated materials must be removed—this guide does not account for such expenses.

• Windows and Doors: In addition to the structure, consider the windows and doors required for your extension. Features like aluminium double glazing, oversized doors, or window seats will increase your budget compared to standard options.

• Decorating: Finally, once your impressive new structure is in place, don’t forget to budget for decoration. Ensure you allocate some of your funds to make the new space look as fantastic inside as it does outside.
   

Extension Costs by Type
 

Shell-Only Extension Cost

A shell-only extension includes the external build components—foundations, brick or block walls, and a roof—providing a watertight structure for future work. This type of extension does not include internal walls, utilities, glazing, or flooring beyond a concrete base. Shell-only extensions are ideal if you plan to handle the non-structural tasks (such as plastering and painting) on your own, or if you wish to complete the project in stages as funds allow. This option also suits those looking to hire a specialist for interior design work. Shell-only extensions generally cost between £1,200 and £1,700 per square metre (m²).
 

Full House Extension Cost per Square Metre

A full house extension provides a finished living space once the work is complete, with internal walls, utilities, flooring, glazing, doors, and more included. While decorative elements may not be part of the package, you’ll have a fully functional room. For a standard full extension, budget around £1,800 to £3,000 per m². Keep in mind that specialised rooms like lofts, basements, kitchens, and bathrooms may have higher costs.
 

Single-Storey House Extension Costs

Ground-floor extensions, typically added to the side or rear of a property, are among the more straightforward extensions, though they still present some challenges. Priced at roughly £1,800 to £3,000 per m², here’s a quick cost breakdown for single-storey extensions of various sizes:

• 20m² Extension: £40,000 – £56,000

• 30m² Extension: £60,000 – £84,000

• 50m² Extension: £100,000 – £140,000

Larger single-storey extensions may benefit from cost reductions per m², reflected in the upper range of these estimates.
 

Two-Storey House Extension Cost

Building a two-storey extension doesn’t necessarily mean double the price, as the foundation and roof are already part of the work. The main additional costs come from constructing new walls, floor joists, and interior fixtures, as well as safe-height working requirements like scaffolding and structural support. If you plan to add a second storey eventually, building both at once can often be more cost-effective. For a typical two-storey extension of 60m², expect costs between £105,000 and £155,000.
 

Additionally, if new legislation is passed, double-storey rear extensions may be allowed without planning permission under permitted development rights, though they’ll still need to meet building regulations. This could streamline the process significantly.
 

Semi-Detached House Extension Costs

Semi-detached extensions are popular and carry similar costs to other extension types, usually between £1,800 and £3,000 per m². If your work is near a shared wall, you may need a party wall agreement, which is an important factor to keep in mind when planning this type of extension.
 

House Side Extension Costs

Side extensions are a versatile option, ideal for expanding existing rooms like kitchens or living rooms or for adding small spaces such as home offices, gyms, or conservatories. You can even opt for a wraparound extension, an L-shaped design covering both the side and rear of your home. This allows you to maximize new space without sacrificing much of your garden. Costs for side extensions generally range from £1,800 to £3,000 per m².
 

Bathroom Extension Costs

Bathroom extensions can either involve adding a new bathroom, such as an en suite or downstairs toilet, or expanding an existing bathroom to include features like a walk-in shower or double sinks. While the space costs are similar to other extension types, the overall cost for bathroom extensions is often higher due to additional plumbing, fixtures, and finishes. You should budget an extra £5,500 to £9,000 on top of general extension costs for a bathroom project.
 

Kitchen Extension Costs

Kitchen extensions tend to be more expensive than standard extensions due to the need for specialised work, including plumbing, gas, and electrical installations. Additional costs may arise if you need a new boiler or if you plan to relocate an existing one. Therefore, you should budget an extra £15,000 to £35,000 on top of the average extension cost for a kitchen project. For more guidance, we offer a range of kitchen extension ideas and planning tips to help you get started.
 

Bathroom Extension - Add £5,500 - £9,000 to total cost (average)

Kitchen Extension - Add £15,000 - £35,000 to total cost (average)
 

Cost of Garage Conversion

Converting an existing garage is one of the most cost-effective ways to create more functional space in your home. You can opt for a partial conversion, which retains some storage space separated by a stud wall. Garage conversions typically cost between £8,500 and £20,000 for a 15m² space, including materials and labour. If you plan to convert the garage into a bathroom or kitchen or if the garage is detached from the house, expect additional costs due to the need for extra plumbing and electrical work.
 

Cost of Loft Conversion

A loft conversion can transform an underused space into a functional room, such as a playroom, bedroom, or office. While it doesn’t involve extending your home’s footprint, loft conversions do require significant work to make the area suitable for living. This includes insulation, structural support, and sometimes scaffolding due to working at height. For a 30m² loft conversion, costs generally range between £50,000 and £95,000.

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Cost of Basement Extension

For those looking to add substantial space to their home without altering its exterior, a basement extension is an ideal solution. Although basement conversions are among the most expensive types of extensions, they offer considerable space and add significant value to a property. A 40m² basement extension generally costs between £100,000 and £160,000.

 

Rear House Extension Cost
 

The cost of a rear house extension will vary based on the size, complexity, and specific design details. Generally, rear extensions cost between £1,800 and £3,000 per m². Projects that involve unique designs, complex structural elements, or particularly large spaces may come with higher costs. Additionally, the total cost is influenced by the quality and quantity of materials chosen, as well as the rates of the tradespeople involved.

Mid-Terrace House Extension to the Rear Cost

For mid-terrace houses, rear extensions are often smaller to medium in size. Generally, you can expect costs in the following ranges:

• Small Extension (15m²): Approximately £30,000 – £42,000

• Medium Extension (30m²): Around £40,000 – £56,000

• Large Extension (50m²): Roughly £100,000 – £140,000
   

Average Cost of House Extensions

Extensions come in various sizes and types, so costs can vary widely. Typically, single-storey extensions cost between £30,000 and £140,000, while two-storey extensions range from £105,000 to £155,000.
 

House Extension Cost Calculator

To estimate your extension cost, simply multiply the cost per square metre by the planned area (m²). For instance, a 30m² side extension could cost between £60,000 and £84,000. These figures consider average market costs, typical construction methods, and standard materials.
 

Keep in mind that factors like specialised construction methods, high-end finishes, or the need for rare equipment will increase costs. Likewise, fluctuations in material prices can also impact your budget.
 

How long will my extension take?

The duration of an extension project varies, but typically, it takes about 3-6 months. Factors like the project’s size, planning permission issues, material availability, and weather conditions can affect the timeline. Delays are common, especially when multiple contractors are involved, so it’s wise to plan for some extra time to complete your build.
 

Should I Extend Up or Out?

Both options come with distinct benefits:
 

• Extending Up:
  Building upwards allows you to maintain outdoor space while gaining additional rooms, such as a loft conversion. This approach can also provide attractive views of the surrounding area. However, it requires space for stairs, which may reduce landing space or require converting part of an existing room.

• Extending Out:
  Expanding into the garden offers greater flexibility in terms of layout and design, allowing you to add substantial square footage and potentially create impressive open-plan spaces. It also creates an opportunity to seamlessly connect your indoor and outdoor areas.

Ultimately, the choice to extend up or out depends on the space you want to create, the limitations of your loft and garden, and your budget.
 

How long does a planning application take to be approved?

Approval times can vary by council, but generally, decisions for household and minor developments are made within the statutory 8-week period. It's best to contact your local planning department for specifics.
 

Should I move out during the construction?

Moving out can make the process easier, as tradespeople won’t need to pause work to accommodate household needs, such as shutting off utilities. However, if you can’t move out, contractors can usually work around you. In these cases, they can set up a sealed work area to minimize dust and debris, ensuring the rest of your home remains comfortable.
 

Do I need planning permission for my extension?

In the UK, many extensions fall under Permitted Development (PD) rights, meaning they don’t require formal planning permission. Under PD, you can:

• Add a single-storey rear extension up to 8m on detached properties and 6m on semi-detached or terraced properties.

• Add a two-storey extension up to 3m, as long as it is at least 7m from the rear boundary.

• Construct single-storey side extensions up to 4m high and no more than half the width of the original building.
   

Always refer to the Government Planning Portal for a full list of regulations and restrictions.
 

Will an extension add value to my property?

Yes, an extension can significantly increase your property’s value. Research by Nationwide Building Society indicates that adding an extra bathroom can increase value by 6%, a double bedroom by 12%, and a double bedroom with an en-suite by 23% on the average three-bedroom home.​