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House Extension Cost Guide

If you’re considering a house extension, understanding the costs involved is crucial. Our guide will provide a comprehensive overview of the expenses, including labor, materials, and any additional fees that might apply. Dive into our detailed house extension cost guide below to learn more about how you can budget and its implications of extending your home.

House extension cost guide

How Much Does an Extension Cost?

The cost of a house extension can vary depending on the size, type, and quality of the extension. To estimate the house extension cost per square metre, one can initially request an estimate. If the estimate is acceptable, an estimator will draw up a final quote that covers the full bill of materials. When deciding whether to proceed with a house extension, it is important to consider the value it will add to the home, as well as any potential extra costs if the project overruns.

The following table provides an estimated cost range for different sizes and qualities of house extensions:

Extension QualitySizePrice Range – Low (exc. VAT)Price Range – High (exc. VAT)Average UK Cost (exc. VAT)
Standard (Small)20m²£40,000£60,000£50,000
Standard (Medium)30m²£60,000£80,000£70,000
Standard (Large)50m²£80,000£150,000£115,000

It is important to note that the cost of a house extension can vary depending on factors such as location, materials used, and complexity of the project. Therefore, it is recommended to consult with a professional estimator to get an accurate quote for the specific project.

House Extension Cost for Different Types

When considering adding an extension to one’s home, it’s important to understand the costs involved. The following sub-sections outline the costs for different types of house extensions.

Single-Storey Extension Cost

A single-storey extension is a popular way to increase the living space of a home, typically used to enhance the kitchen or living room on the first floor of a property. The cost of a single-storey extension installation is around £1500m² to £2500m². The cost varies depending on the size and quality of the build. The table below sets out the single-storey extension cost for different sizes of basic, average, and premium quality builds.

Size (m²)BasicAveragePremium
Small (15m²)£26,250£31,500£37,500
Medium (25m²)£43,750£52,500£62,500
Large (50m²)£87,500£105,000£125,000

Double-Storey Extension Cost

The cost varies depending on the size and quality of the build. The following table lays out the costs for different sizes and qualities of double-storey extensions. A double-storey extension is built across two floors of a property, increasing the available living space by allowing the addition of multiple rooms. The cost of a double-storey extension is between £1500 to £2450 per square metre.

Size (m²)BasicAveragePremium
Small (30m²)£51,000£61,500£73,500
Medium (60m²)£102,000£123,000£147,000
Large (100m²)£170,000£205,000£245,000

Rear Extension Cost

The cost of a rear extension is around £1500 to £2500 per square metre. The cost varies depending on the size and quality of the build. The following table sets out the prices for different-sized basic, average, and premium rear extensions. A rear extension is installed at the back of a property where there is a large garden area.

Size (m²)BasicAveragePremium
Small (15m²)£26,250£31,500£37,500
Medium (25m²)£43,750£52,500£62,500
Large (50m²)£87,500£105,000£125,000

Side Extension Cost

Side extensions are a popular choice for properties with a lot of space on the left or right-hand side of the home. Typically, side extensions cost around £2000 to £4000 per m². The cost varies depending on the size and quality of the build. The following table shows the different-sized buildings for basic, average, and premium side extensions.

Size (m²)BasicAveragePremium
Small (20m²)£40,000£60,000£80,000
Medium (40m²)£80,000£120,000£160,000
Large (60m²)£120,000£180,000£240,000

Side-Return Extension Cost

The price of a side-return extension per square metre is around £2000 to £4000. The cost varies depending on the size and quality of the build. The table below sets out the different costs of basic, average, and premium side-return extensions. Properties with a small spaces at the side of their home which is used to store bins, bikes, and more may benefit from a side-return extension.

Size (m²)BasicAveragePremium
Small (6m²)£12,000£18,000£24,000
Medium (9m²)£18,000£27,000£36,000
Large (12m²)£24,000£36,000£48,000

Kitchen Extension Cost

A kitchen extension is a great way to have a larger kitchen in your home. The cost of a kitchen extension is around £1500 to £2500 per square metre plus £200 to £1000 per m² for the actual kitchen installation itself. The cost varies depending on the size and quality of the build.

Types of House Extensions

There are several ways to extend a home, including single and multi-storey extensions, semi-detached extensions, kitchen and bathroom extensions, garage conversions, loft and basement conversions. Each type of extension has its own unique benefits and considerations, such as cost, planning permission, and potential increase in property value.

Additional house extension cost to consider

House Extensions Surrey | Single Storey Extensions Surrey | A-Spec Ltd
House Extensions Surrey | Single Storey Extensions Surrey | A-Spec Ltd

When building an extension, there are additional costs to consider besides the actual construction. These costs can include architect fees, groundwork costs, planning permission fees, and building regulation costs.

Extension Architect Costs

Hiring an architect to design and plan your extension can ensure that it meets building regulations and is installed properly. The cost to hire an architect varies but is typically around £50 to £100 per hour.

Extensive Extension Groundwork Costs

Before building an extension, it is essential to prepare the ground properly. This can include excavating uneven ground, which can cost around £30 to £50 per cubic metre. Installing new drainage may also be necessary, which can cost around £800 to £3000.

Extension Planning Permission Costs

To avoid additional expenses, it’s crucial to align your extension project with planning permission guidelines. The cost for a planning permission application is approximately £200. To steer clear of extra charges, ensure that your extension doesn’t occupy more than half of your property’s land area, doesn’t surpass the existing roof’s peak, and doesn’t incorporate raised platforms, balconies, or modifications like chimney flues, vent pipes, or any roofing work.

Extension Building Regulation Costs

Most house extensions require building regulations approval before construction can begin. The cost of building regulations approval varies depending on the local council. Various elements of an extension must be signed off by a professional to ensure the property is safe, including internal and external walls, flooring, ventilation, and foundations. The cost of these elements can vary depending on the specific needs of the extension.

Overall, it is important to consider these additional costs when planning an extension project. Proper planning and preparation can help ensure that the extension is installed properly and meets all necessary regulations.

What are the Costs of Supplies for an Extension?

For those planning a DIY extension or purchasing supplies themselves, the following costs should be considered:

  • Bricks: Prices range from £400 to £1000 per thousand.
  • Timber: Prices vary depending on the type, but expect to pay around £20 to £40 per square metre.

It’s important to note that these prices are subject to change and may vary depending on location and supplier.

Tradesmen Costs for an Extension

Carpentry Contractors Surrey | Carpentry Company Surrey | A-Spec Ltd
Carpentry Contractors Surrey | Carpentry Company Surrey | A-Spec Ltd

When planning a house extension, it is important to consider the cost of hiring different tradespeople who will be involved in the construction process. Here are the estimated costs for each trade:

Extension Builder Cost

Builders are responsible for constructing the foundations and the actual structure of the extension to ensure it is robust and stable. The average cost to hire a builder for a house extension is around £250 to £450 per day.

Extension Plumber Cost

If building a kitchen or bathroom extension, a qualified plumber is required to install sinks, toilets, appliances, and more. Plumbers cost around £250 to £350 per day.

Extension Electrician Cost

To get the extension fitted with lighting and electrical appliances, a professional electrician is required to ensure everything is compliant and working properly. The average hourly rate for an electrician is typically around £30 to £60 per hour or £250 to £350 a day.

Extension Plasterer Cost

To prepare the walls and ensure any paint or wallpaper lasts, an expert plasterer is required. The estimated cost of hiring a tradesman to plaster a new extension is around £200 to £300 per day.

Extension Painter and Decorator Cost

After the plastering is complete, a decorator is required to paint or wallpaper the internal and external walls of the new house extension. For multiple rooms, the estimated cost is £200 to £300 a day.

Extension Carpenter Cost

For a new extension, a professional carpenter is required to help with the construction and installation of the new extension. On average, a carpenter costs around £250 to £350 per day.

Extension Kitchen Fitter Cost

For a new kitchen extension, it is recommended to employ a kitchen fitter to install the kitchen cabinets and worktops. Average day rate of between £250 to £350.

Extension Window Fitter Cost

After the extension is built, a professional window fitter is required to install fully insulated windows. For one or two windows, the estimated cost is around £25 to £35 an hour. For a full set of windows on a double-storey extension, the estimated cost is between £250 and £350 per day.

Extension Labourer Cost

A labourer is required to assist with the majority of jobs onsite, including building foundations, the extension structure, and more. The estimated day rate for a labourer is £120 to £180 a day.

Extension Bricklayer Cost

To ensure the extension is built properly, an experienced bricklayer is required. Bricklayers tend to charge around £250 to £350 a day.

It is important to note that the costs provided are only estimates and may vary depending on the location, experience, and quality of the tradespeople hired.

How Long Does It Take to Build an Extension?

Building an extension can be a time-consuming process. It’s important to understand the timeframes involved in building the type of extension you want.

For a single-storey extension, it typically takes 2 to 3 months. A double-storey extension can take 4 to 6 months.

A side extension takes 2 to 3 months, while a side return extension takes 1 to 2 months.

A rear extension takes 2 to 4 months, and a wrap-around extension takes 4 to 6 months. A kitchen extension can take 2 to 6 months, and a bathroom extension takes 2 to 4 months.

A prefab extension takes 8 to 10 weeks, and a porch extension can take 3 to 14 days.

Looking for house extension services? Get a free quote from us.

A-Spec Ltd are construction and carpentry contractors based in Surrey providing  all types of house extensions across the South East. Get in touch with for a free estimate.

How Much Does Each Stage of an Extension Cost?

Building an extension involves various stages, each with its own costs that need to be considered when budgeting for the project. The following sub-sections detail the costs associated with each stage.

Building Regulations & Design

The first stage of building an extension involves ensuring that the design complies with current building regulations and planning permission rules. If the initial plans do not adhere to the conditions, the homeowner will need to change the plans or pay anywhere from £200 or more to apply for approval. Once approval is obtained, an architect is needed to draw up plans for the structure.

Foundations and Groundworks

Before any building can begin, the ground must be fully prepared. This involves excavation, which will vary in cost depending on the land. Pipework may also need to be removed and new drainage installed, costing between £1000 to £5000. Once the groundwork is complete, the foundations can be set to provide a stable framework for the installation of the extension. Installing foundations is a time-consuming component of the project.

Building the Outer Shell

After the ground has been prepared and the foundations have been set, the next stage involves building the outer frames and walls using timber, bricks, blocks, and tile materials.

Fixtures and Fittings

The final stage involves the finishing touches, which includes installing furniture such as kitchen cabinets and worktops. The cost for this ranges from £5000 to £25000. For a bathroom extension, the cost for brand new bathroom fixtures ranges from £1500 to £20000. Windows and doors are also installed at this stage, with the cost for window installation ranging from £350 to £1200 per window, and door installation typically priced at £50 to £150 per door. Indoor and outdoor light fixtures can also be fitted at this point, with a cost ranging from £100 to £1000 when fitted by a professional electrician.

In conclusion, building an extension involves several stages, each with its own costs. Homeowners should carefully consider each stage of the project and budget accordingly to ensure that they can complete the extension within their financial means.

Factors Affecting the Average House Extension Cost

Constructing home extensions is a complex task that involves building the structure and making it fit for living. The biggest cost of an extension is usually the build cost, which includes materials and labour. However, several other factors can affect the cost of a house extension.

Size of Extension

The size of the extension is a significant factor that affects the cost of the project. Smaller, less complex extensions will cost less than those with grander aspirations. The average cost of a 20 m² extension is £50,000.

Location of Home

The location of the home is another factor that can affect the cost of a house extension. Builders in London and the South East of the UK typically charge more than those in other locations. However, the costs in this guide are not location-based, so homeowners should consider this if they live in one of these areas.

Ease of Access

Builders will need easy access to the work site for tools and machinery. For this guide, we’ve assumed a good level of access. However, if the site is difficult to access, the cost of the project may increase.


The quality of the extension is another factor that can affect the cost. Depending on the materials used, the complexity of the build, and the skill of the contractor, homeowners can expect to pay more or less than the market rate.

Planning Permission

Planning permission is another factor that can affect the cost of a house extension. Not all extensions need planning permission as they fall under permitted development rights. Homeowners should check with their local council to see if planning permission is required.


Using an architect can help streamline the building of an extension in many ways. They can come up with the perfect design that suits the homeowner’s requirements, secure accurate quotes, and aid with the planning permission process. However, they work on either a percentage rate based on the total build cost or a flat fee.

Finishings and Fittings

The finishings and fittings of the extension should be factored into the total cost of the project. From beautiful lighting to bespoke flooring and joinery, how the room will look can affect the cost. Homeowners should consider their preferences when budgeting for the project.


Groundworks are an essential task that ensures the structural integrity of the extension. There can be additional costs depending on soil type, if drains need moving, and if any tree roots need removing. For this cost guide, we’ve assumed foundation depths are normal, and there are no complications.


Homeowners should tell their home insurance company of any planned development work before it starts. This will likely increase their premium but will be necessary to ensure they’re covered during the development work and that their new extension is included in the policy once complete.

Clearing the Space

Clearing the space is another factor that can affect the cost of the project. From removing trees and shrubbery to dismantling awnings and patios, there’s often a lot to be done to make the space fit for purpose before any work can begin. Depending on the extent of work here, the cost can increase.

Windows and Doors

In addition to the structure, homeowners also need to think about the windows and doors they’ll need. If they want double glazing, bay windows or window seats, then they’ll have to budget more than for standard windows.


Finally, once the extension is complete, homeowners will need to decorate it. They should save some of their budget for making the new space look as great on the inside as it does on the outside.

Looking for a contractor for your next house extension project?

When searching for a contractor for your house extension project, consider the following steps:

  1. Research Contractors: Look for contractors with experience in home extensions. You can start by asking for recommendations from friends or family who have had similar work done, or search online for local contractors with good reviews.
  2. Check Credentials: Ensure the contractor is licensed, insured, and has the necessary permits to work in your area. Check their professional background and any memberships in trade associations.
  3. Review Past Work: Ask for a portfolio of their previous projects. This will give you an idea of their work quality and style.
  4. Get Multiple Quotes: Obtain quotes from several contractors to compare prices. Remember, the cheapest option may not always be the best in terms of quality.
  5. Ask for References: Contact previous clients to get feedback on their experience with the contractor.
  6. Discuss Your Project in Detail: Clearly communicate your vision, timeline, and budget. Make sure they understand your requirements and expectations.
  7. Review the Contract Carefully: Before signing, thoroughly read the contract. It should include details about the scope of work, timeline, costs, payment schedule, and warranty.
  8. Plan for Contingencies: Be prepared for potential delays or unexpected costs. A good contractor will be transparent about possible challenges.
  9. Consider Communication Style: Choose a contractor who communicates well and is responsive to your queries and concerns.
  10. Check for Planning Permission Knowledge: Make sure the contractor is familiar with local planning permission regulations, especially if your project requires such permissions.

Remember, a good contractor is not just someone who does quality work, but also someone who understands your vision and communicates effectively throughout the project.

Here at A-Spec Ltd, we are fully accredited house extension builders covering residential and commercial projects across South East England. Contact our management team for a quote and consultation today.

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A-Spec Ltd is a Surrey-based carpentry contractor offering residential and commercial services to contractors, house builders, architects, businesses, and private clients across the South East.