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Prefab Extension Cost: 2023 Modular Kit Prices UK

If you own your own home and have a family, you’ll know that space becomes tighter and tighter as the kids grow up. Eventually, you have no choice but to move house.

But that’s not your only option!

You can affordably increase your living space by choosing an instant home extension.

In the UK, an average prefab extension costs about £25,000-£33,500 for a single storey module of around 20m2. However, if you prefer a smaller or larger prefab room extension, they are available for various amounts, but these we’ll consider later.

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A prefabricated house extension is a flatpack kit made in a factory to your measurements, transported to the site and assembled on pre-existing foundations. Then, suitable tradespeople finish off the construction to provide you with a home extension at a fraction of the cost of a traditionally built structure.

Are there any other reasons why you should install a prefab extension to your house rather than move?

Moving costs are a lot higher now than when you moved into your home. Add together stamp duty, legal fees, removal firm costs, estate agent fees and all the surveys you need. Not to mention redecorating the new house and getting the garden just how you want it. All these add up very quickly and will probably go a long way towards paying for a new modular house extension.

How Much Does a Prefabricated Extension Cost?*

In the table below, we’ve outlined the prices of different sized prefab extensions:

Extension SizeFinishEstimated Cost
Small (20 m2)Basic£25,000-£50,000
Small (20 m2)Average£25,000-£50,000
Medium (30 m2)Average££37,000-£68,000
Medium (30 m2)Premium£52,000-£100,000
Large (50 m2)Average£62,000-£113,000

*The figures shown in the table are based on average estimates (including VAT) drawn from online resources. Every home is different, as is every prefabricated extension. Therefore, use the estimates as a starting point when researching your project.

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Adding a Bathroom or Kitchen?

If you decide to install a kitchen or bathroom in your extension, plumbing, drainage, fixtures, and appliances will incur additional costs. The table below highlights some of the costs.

ItemEstimated Cost
Flooring Materials (Average Price)£50/m2
Kitchen Appliances (Full Range)£1,500-£2,000
Kitchen Units (Medium Sized Kitchen)£1,500-£3,000
Kitchen Fitting (Average Price)£4,500-£6,000

The prices shown in this table are for average level appliances and finishes. Alternatively, you can choose budget or premium ranges to decrease or increase the additional costs.

Other costs

You must also take into account other costs when planning your project. The following table highlights the most important.

ItemEstimated Cost
Wall/floor tiling£20/m2
Install plumbing/heating system£150-£350/day
Rewiring house£150-£350/day
Structural repairs£150-£300/day

These costs vary depending on the work required to complete the job and whether the tradespeople work solely on the prefab extension or incorporate work in the main house.

The scaffolding rates will vary depending on the height. That is, whether you have a prefabricated second storey extension on top of the ground floor.

Prefab Extension Cost Factors

Various factors affect the cost of your prefab extension.


Usually, the most significant expenditure is the build cost which incorporates materials and labour. Small extensions with a less complex design will cost less than a larger or more complex design.

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Labour rates in London and the southeast of England are higher than elsewhere in the country. Usually, these labour rates can be as much as 20% more.


Builders always need good access to the worksite, even more so if they need access to the site for lorries and mobile cranes.


Your choice to use premium or budget materials, and the contractor’s skill level, will always affect whether you pay more or less.

Planning Permission

Some, but not all, extensions require planning permission. It depends on whether your project complies with the requirements for permitted development rights. Moreover, some designated areas of the country have stricter permitted development rights than others. Therefore, you should find out what restrictions are in force where you live. If you end up having to submit planning applications, you will be charged a fee. Charges for a house cost about £460, whereas an extension costs around £200. Contact your local authority to find out the charges in your area.


Whereas the prefab manufacturer will design and draw the plans for the extension, you will need an architect to design the entire project and submit a planning application if necessary. If you need structural alterations on your house to accommodate the extension, you will also need a structural engineer. Professionals such as these usually charge a flat rate or a percentage of the total project cost.

Fixtures, fittings and finishes

If you’re spending good money on a prefab extension, you also need to include the extras such as lighting, plumbing, flooring, doors and decorating. Don’t forget to incorporate these costs into your overall project.


Many people forget the essential part of the project. Every extension needs foundations on which to stand. Moreover, it also needs a concrete floor. Groundworks can vary considerably depending on how the local geology and water table behaves. Furthermore, it isn’t always possible to predict these factors before the groundworkers start to dig. So be prepared to pay more for what you thought might be a cheap and straightforward job.

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Building a Prefabricated Extension: Step by Step

Every house is different, and the design of each prefab extension depends on many factors. Therefore, It’s impossible to give an accurate build description. Instead, we’ll take you along the significant steps, describing what you need to know and do.


You’ve probably drawn a little sketch on the back of an envelope. Unfortunately, this isn’t good enough for the local council planning department or the building regulations.

You’ve chosen a prefab extension, so talk to the company about a drawing when you ask for a quote. Additionally, the extension also needs foundations, a floor and some way of connecting to the existing house. You’ll need an architect to design this, and they’ll need the drawing from the prefab company to incorporate into the plans. Connecting to the house might need structural alterations, so you need a structural engineer to provide plans and calculations for the building regulations. Moreover, you also need construction drawings so the building contractor knows what to do.


Make sure there is room for the contractors to work. Furthermore, they need room for delivery vehicles and a crane. Before work starts, you must also decide where the contractors can park and store tools. Finally, remember that contractors must have somewhere to wash their hands and use a toilet. You can hire portable washrooms from your local tool hire shop.


Your new prefab extensions will need electricity and heat at a minimum. If it is a kitchen or bathroom, you’ll also need drainage and a water supply. So, before you start, ask qualified tradespeople to check that your existing electrics, heating, drainage and plumbing can cope with the additional use. If they can’t, then you must upgrade during the project.


Notify your home insurance company of what’s going on, and check you’ve insurance for any issues during the project. Not only that, but the insurer might have special extension policies that you can take out. You should also notify your mortgage holder because your property’s value will increase.

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Planning permission and building regulations

Finally, before you start, make sure the architect has told the authorities what’s going on. You might need planning permission for your project depending on whether it complies with the council’s permitted development rights. But, whatever happens, you will need to comply with building regulations.

Project managing the extension

You can use the company that designs and builds the prefab, or you can use an architect. Alternatively, when building foundations and when the prefab connects to the house, use the main contractor, who has a team of other tradespeople.

Remember that the work must comply with the building regulations. Therefore, the local authority’s building control inspector must monitor each build stage to ensure compliance. Alternatively, hire tradespeople who belong to the UK Government’s competent person scheme.  This register allows members to self certify their work as compliant. You will be issued a certificate by each registered member to state that their work is up to standard. The scheme members also notify the local authority of the work done.

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Do I Need an Architect?

An architect is invaluable when building an extension. The prefab manufacturer only has responsibility for their product. You have to supply the foundations, concrete floor slab, and organise the connection between the prefab and the house. Depending on your preferences, this can be as simple as using an existing external back door. Alternatively, you might have to remove an entire wall and provide structural strengthening and lintels. Therefore, you need an architect who will also use a structural engineer. Together, they will design the foundations and how the prefab joins the house.

Prefab Extension Benefits

There are many benefits to using a prefab extension kit. Let’s have a look at the most important:

Speed of construction

Prefabs are very easy to erect. Typically, once the foundations are in place, a prefab can take about 6-8 days. The quick construction time, in turn, means that there is less disruption and mess.


Many manufacturers offer a DIY option. The kit arrives, complete with installation instructions and demonstration videos.


Because the prefab uses lightweight materials, excavation and foundations don’t have to be heavy-duty; Similarly, it’s easy to build over waste pipes and existing drains.


All prefab extensions have manufacturer’s guarantees. Often, the warranty period lasts for twenty years for structure and roof.

Bespoke manufacture

There are many ready-made designs that you can choose from. Then, you only need minor alterations for the prefab to match your unique circumstances. Finally, you’ll have a prefab explicitly built for your house’s measurements.

Does a Prefab Extension Add Value?

A typical prefab extension added to your home will increase its value by up to 23%. Furthermore, if you’re adding a kitchen or bathroom, then you can add an extra 6% onto this.

However, if you intend to sell your home shortly, get advice from a local estate agent who knows the value of homes in your area. Then, you won’t spend more on your project than is necessary. The last thing you need to do is waste money by not getting a return on your investment.

Prefab Extension Types

Timber frame extension cost

Timber is one of the most common building materials used in prefab extensions. The material usually costs around £70/m2 and provides structural strength. Furthermore, plywood sheets fixed to studs make the panels. Then, they’re insulated and wrapped in a vapour barrier, before having an external cladding added.

Timber prefabs are very fast to construct in the factory and assemble on site. They’re also more sustainable than most other types. The waste wood generated in the factory makes manufactured boards such as chipboard and MDF.

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Unfortunately, timber frame panels are expensive. Moreover, the panels don’t absorb sound very well, which means that you might need extra insulation.

Steel prefab extension cost

This type of prefab extension is made from lightweight steel with insulation fixed outside a masonry frame. Typically, this costs between £35 and £40/m2. The design of the lightweight steel and insulation prevents cold bridging and condensation.

Alternatively, a galvanised steel prefab costs from £40 to £200/m2. Probably, the best advantage of using galvanised steel is its corrosion resistance. In everyday use, it’s possible to have a lifespan of up to 60 years or more.

SIPs prefab extension cost

SIPs, or Structural Insulated Panels, use high-quality foam insulation sandwiched between two structural engineered-timber boards. Usually, these cost around £80/m2, so they don’t cost much more than other prefab materials. However, they are very robust and often used for many different purposes. Furthermore, they aren’t susceptible to pest infestation because they have a thick layer of foam between the inner and outer skins. Unfortunately, structures made from SIPs need more ventilation because the foam insulation traps too much heat inside the house. Furthermore, the timber boards tend to rot over time, no longer bonding with the foam. Therefore, the prefab loses its structural integrity.

Planning Permission & Building Regulation

We’ve already talked a bit about planning permission and building regulations. If your project complies with your area’s permitted development rights, you won’t need to apply for planning permission.

However, your prefab extension must comply with the building regulations. All reputable companies that produce prefab extensions will ensure that their product complies with the appropriate regulations. But, there are other parts of the project that also need compliance. These are foundations, roof, flooring, drainage, ventilation, energy efficiency, electrical work, plumbing, glass safety, fire safety, and the structural opening between the extension and house.

Party Wall Agreement

In addition to the statuary obligations we just mentioned, your project might have to comply with the Party Wall Act 1996. In a nutshell, the Party Wall Act provides procedures to prevent and resolve disputes between you and any neighbour who might object to an excavation or extension that encroaches onto a party or boundary wall.

According to the UK Government website, the Act covers:

  • Any new building that lies on the boundary of two properties.
  • Work that affects an existing party wall.
  • An excavation near to or below the foundations of your neighbour’s buildings.

Therefore, you can see that your project might or might not apply to the Party Wall Act. But if it does, then you must arrange a Party Wall Agreement with your neighbour to ensure, at a later date, that they don’t object to your extension. A qualified building surveyor must draw up the agreement, and their fee for a typical Party Wall Agreement for an extension ranges from £1,200 to £1,500.  Moreover, you and your neighbour have the option to agree to use a single surveyor to handle the agreement for both parties or use one surveyor each. In the latter case, the costs will be accordingly higher than when using a single surveyor.


Choosing a prefabricated extension can provide extra room for you and your family without going to the trouble of moving house and at less cost than building a traditional brick and mortar extension. However, there are many things you should know before you sign on the dotted line. You must understand the law and appreciate which is the best prefab for your requirements. Not only that but, if you aren’t careful and haven’t been guided along the way by a professional, prefab extension costs can escalate very quickly.

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