Introduction

Modexa build are experts in the installation of modular extensions. In this guide to modular extensions, we aim to give you all the information you need about modular builds.

If you are considering extending your house, or if you are simply curious about modular builds, this guide should give you all you need to know prior to speaking to our team about your project!

A guide to modular extensions: Context

Modular builds are an increasingly popular choice for building homes and extensions in the UK. This is because they bring a number of tangible benefits.

Tens of thousands of new homes are being constructed every year as modular buildings. Within the next ten years, this is expected to top 50,000 every year; on the continent too, modular buildings are now firmly established as a cost effective, sustainable and versatile solutions.

For those wanting to stay in the location they love, but are looking to add space, modular extensions are a brilliant alternative to traditional builds.

What are modular extensions?

The key difference between a traditional extension and a modular extension is that the main components of a modular extension are manufactured off-site, and only brought to site for assembly when they are needed. These components include walls and ceiling panels.

In effect, using a modular build enables the project to be run simultaneously on-site and in factory. So, a project can be completed in around 50% of the time of a traditional build.

The design and manufacturing is carried out by specialists in precision-controlled environments in the UK. This results in components that fit perfectly first time, every time.

Modular extensions are permanent structures, and in most cases have a higher structural integrity as a traditional extension due to the fact they must be designed to be transported and lifted into place without flexing . They look and feel the same, will be rendered (or alternatively finished) in the same way as a traditional build. The thermal performance of a modular extension outperforms a traditional build by some margin.

Modular builds are made bespoke for every project. At Modexa, we will ensure that your vision for your extension is captured in detail, and that precise measurements are shared with the manufacturers. 

In terms of foundations, all modular extensions are designed such that they can be seamlessly installed on pre prepared foundations where block and brick will be laid to a damp proof course. 

Single Story extension foundations 

Based on a standard foundation design of 600mm wide and 300mm thick concrete foundation. 1000mm to bottom of foundation. Any further mass concrete required to be taken down to a natural bearing strata will be by building control or an engineer’s request.

Double Story extension foundations 

Based on a standard foundation design of 750mm wide and 300mm thick concrete foundation. 1000mm to bottom of foundation. Any further mass concrete required to be taken down to a natural bearing strata will be by building control or an engineer’s request. 

Alternatively ground screw piles can be used to overcome difficulties like drainage and tree roots.

What is the difference between modular and prefabricated?

Both types of construction are essentially pre-fabricated in factory conditions. However, where ‘prefab’ would usually refer to temporary structures, ‘modular’ are very much permanent. They have all the properties of traditional extensions including durability, insulation and aesthetic. 

Modular Vs traditional

Whilst the end product is the same, a modular extension has many benefits over traditional. Here are some of these benefits:

• Precision manufacturing means a perfect fit between components. Not only does this reduce assembly time, it also gives modular builds brilliant insulation properties.
• Precision manufacturing also means that the supply chain for any project becomes much more easy to manage and predict. Traditionally, glazing might have to wait until the structure is complete; with modular, the dimensions for glazing can be predicted from the outset.
• As modular buildings are made in factories and assembled on-site, their readiness can be estimated to the day, or even the hour. This means that extensions can be built much more quickly than traditional builds.
• Disruption on-site is greatly reduced. Assembly can be co-ordinated to correspond to the delivery of other components, reducing waiting time.
• Modular extensions have a reduced carbon footprint due to their mass-production, reduced waste and reduced assembly time.
• The precision of the manufacturing process means that when it comes to, for example, glazing, modular builds will fit perfectly every time.

Here’s a summary of the benefits of modular compared to traditional builds:

Minimising lead times and disruption

The supply chain for traditional building work is often complex and unpredictable. Many different types of materials, from different suppliers and with different sub-contractors need to be co-ordinated in detail.

It only takes one supplier (or a change in the inclement British weather) to delay the entire project.

For every day that passes, the amount of disruption increases and your ‘due date’ will be pushed back.

The beauty of a modular build is that the key elements of the build are ready when they are needed. So, the supply chain is simplified, variations are reduced and co-ordination is easier.

Your extension will be ready when it is needed;

You garden room won’t miss the summer season;

Your granny annex will be finished more quickly;

Your assisted living space will be ready in a much more predictable timeframe.

Cost

The finished cost of a modular extension is comparable to a traditional extension. 

Currently, the cost of materials is higher than with traditional builds, whilst the cost of labour is less, due to the speed with which modular builds can be completed.

However, as modular builds are gaining in popularity, we are already seeing material costs reducing due to economies of scale.

Due to the more predictable nature of modular builds, ancillary items such as skips and scaffold will be needed for less time. Predictability also means that the risk of hidden costs is greatly reduced too.

Aside from the actual build cost, modular builds have superb thermal performance. They require less heating than traditional builds, ensuring you can save money on energy bills down the line.

Cost can also be saved when it comes to design. 

With a conventional build, clients are typically required to appoint architects for detailed building regulation drawings, along with structural engineers to produce calculations—often resulting in additional time, cost, and coordination.

With modular builds, there’s no need for building regulation drawings; The manufacturing drawings we need to build the modular will have already been approved by Building control designed by our in-house engineering team and passed and approved by LABC and NHBC.

All we need to get started is the style and layout of your project. Planning drawings are sufficient to move forward (and we can produce these for you if required), removing the need for separate, costly design stages.

Behind the scenes, our system is fully engineered and pre-designed. Our manufacturer’s technical drawings are not only used for production but are also compliant with building control requirements, meaning they are accepted for approvals and final sign-off.

In addition, all structural elements are calculated and verified by our in-house engineering team as part of the package—eliminating the need to appoint external engineers.

The result? A faster, more streamlined process with fewer consultants involved, reduced costs, and complete confidence that your extension meets all required standards.

As every build is unique, Modexa will need to consult with you directly in order to provide you with an exact, no obligation quote. However, see our guides for an estimated cost, and detail of what is and isn’t included.

How are Modular Extensions Built?

Modexa build will project-manage the entirety of the build process.

How our design and build process works:

Step 1: Consultation

Any project starts with a chat about your potential project. As a significant lifestyle decision, it’s useful to discuss, for example, What do you want from your extension? How is it going to improve your living space? What are the rules around what you can and can’t build?

As we have many years of experience, we can give you insights into things you might not have considered, from layout to finished look and feel.

Step 2: Site Assessment

In order to provide you with an accurate quotation, and discuss your vision in situ, we will visit the site of the proposed build and consult with you about what can be achieved. Again, we are able to make suggestions that will enhance the final build based on our years of experience.

Step 3: Design and Quotation

We will provide you with a tailored proposal resulting from our consultation. The proposal will be fully costed and give you accurate information around delivery and lead times. It will include all the information you need to proceed, including specifications and technical drawings. It will include options depending on your budget around, for example, fixtures, fittings and finishes.

Step 4: Off-site Construction

Whilst the superstructure is being manufactured off-site with our manufacturing partners, we will be preparing your site for delivery, so that, on delivery, everything goes as smooth as possible.

Step 5: Delivery and Installation

With a fully-prepared site now in place, all modules will arrive ready to assemble, all managed with our haulage partners. Once transported, the components will be fully assembled within days. All requested finishes will be completed according to your specifications.

Different types of modular builds

At Modexa, we are committed to enabling you to love where you live! We tend to specialise in five different types of extension. These are:

House extensions Increasing the space in your home to ensure you have the extension of your dreams. Extensions include single storey, two storey and wrap-around

Garden rooms Multi-use stand-alone new spaces that get the most from your outside area

Granny annexes Multi-function spaces that can be completely self-contained

Conservatory conversions Bring your outdated conservatory bang up to date in terms of aesthetics and energy performance

Assisted living spaces Brilliant solutions for lesser-abled family members that can attract funding towards cost

How long does it take to build a modular extension?

A huge advantage of a modular extension is that it greatly reduces the time needed for the build. This is because of the predictable nature of the off-site manufacturing process.

Whilst all projects will vary, once planning is granted the extension will be on site 6 weeks from the order. Modexa will prepare the site 1 week earlier. So in general at Modexa we will have started and finished every project within an 11 week period from planning approval.

Designed and built in the UK

Modexa are proud to work with UK based manufacturing partners. Not only does this help the local economy, it also means communication and co-ordination is straightforward. We have built up these relationships over the years, to the point where we are a trusted supplier, working with trust and transparency.

All Modexa builds are completed to current UK building regulations.

Which is the best material to use for modular builds?

Light Gauge Steel (LGS): is a purpose-built material for modular builds that produces cold-formed, thin sections of steel. These are engineered by manufacturing partners to precise tolerances, and will form the basis of the superstructure (walls, floor and roof) prior to being transported to site for assembly. Typically 3mm thick cold rolled into structural C sections, LGS is galvanised structural steel in sections that lock together for a brilliant fit.

Light Gauge Steel has a number of properties that lends itself to modular builds:

• Very high strength to weight ratio
• Unbeatable accuracy in terms of fit, both to other sections and to insulation and cladding
• Extremely durable due to galvanised coating
• Relatively quick to produce
• Not susceptible to any rot/fungi associated with other materials especially wood

Both steel and LGS have enormous structural integrity, and thus can be fitted at the manufacturing site with insulation and cladding prior to delivery.

Sustainability and performance

Modular construction delivers strong sustainability and building performance because most of the structure is manufactured in a controlled factory environment rather than built entirely on site. This approach significantly reduces material waste, improves quality control, and shortens the duration of construction projects.

From a sustainability perspective, modular builds typically generate 30–60% less site waste, require fewer site deliveries, and reduce overall construction emissions due to the efficient manufacturing process. The controlled production environment also allows materials to be used more efficiently and enables the integration of modern low-carbon components such as high-performance insulation and airtight construction systems.

In terms of performance, modular buildings are designed to achieve excellent thermal efficiency and airtightness, often exceeding minimum building regulation standards. The precision manufacturing process ensures tighter tolerances than traditional construction, which helps reduce heat loss and improves overall energy efficiency. This results in lower heating demand, improved occupant comfort, and reduced operational energy costs over the life of the building.

Combined with faster build programmes, reduced disruption on site, and consistent quality, modular construction provides a sustainable and high-performing alternative to traditional building methods.

Here’s a breakdown of what we call ‘High-Performance Living’:

EPC Performance

Modexa homes are built using our modular systems and are designed to achieve very high EPC ratings, typically A, thanks to precision factory manufacturing, high insulation levels and reduced thermal bridging.

Higher EPC ratings mean:

• Lower energy bills
• Reduced carbon emissions
• Increased property value
• Better mortgage and lending options

Air Tightness & Thermal Performance

Because Modexa modular homes are manufactured in controlled factory conditions, construction tolerances are far tighter than traditional building.

This results in:

• Significantly reduced air leakage
• Improved thermal efficiency
• Consistent insulation installation
• Lower heating demand

Typical modular airtightness levels can be 70% better than many traditional builds

Renewable Energy Ready

Modexa Modular homes are designed to integrate seamlessly with modern renewable technologies such as:

• Solar PV systems
• Air source heat pumps
• Mechanical ventilation with heat recovery (MVHR)
• Smart energy systems

This ensures your home is ready for the future of low-carbon living

Traditional construction:
Variable performance depending on site workmanship.

Modular construction:
Factory precision delivering consistent thermal and airtightness performance

Finishes

We offer a full range of interior and exterior finishes, depending on whether you want your extension to blend in with your existing property, or stand out and make a statement! We have experience of a number of aesthetic embellishments depending on your taste. Here are just some of our options:

Brick slips. A brick slip is a brick veneer made from real brick, usually 15-30mm thick, which create the same visual and tactile impression of brick, but without the cost or weight. By using brick slips, we can match any brick type in the country, ensuring a perfect match to any brickwork you may have on your existing dwelling. 

Render. We can access a huge range of renders to give you the finish you want. Our render is eco rend accredited and gives a brilliant and durable finish.

Aluminium Cladding. The addition of aluminium cladding to a modular build gives a brilliant, durable and decorative finish. Aluminium cladding is ultra-low maintenance, fire resistant and extremely versatile. It lends itself brilliantly to modular builds, as it is easy to install and cut to precise tolerances.

Zinc Cladding. By introducing zinc cladding to the outside of your building, you can ensure a beautiful, contemporary finish that is highly weatherproof, does not corrode and will not degrade from expose to UV. Zinc cladding has a lifespan of up to 100 years, even in harsh climate conditions. Zinc is also extremely malleable, so can be formed into an infinite number of shapes, giving your project a boutique, bespoke feel.

Timber cladding. For a luxurious finish, we can offer timber cladding in both oak and cedar.

Finally, by combining two or more different materials, we can give your modular extension a unique, bespoke and breathtaking finish!

If you have found this guide useful, why not also take a look at our Guide to Building Regulations here?