Offsite modular construction has emerged as a popular solution to tackle the chronic housing shortage and attain net-zero carbon targets in the UK. This technique involves the prefabrication of building components, sub-assemblies, and parts in off-site factories, which are then transferred and assembled on site as integral parts of a larger structure. These modular units may either constitute a small fraction of the overall project or form the entire building. Accordingly, the proliferation of modular construction is frequently justified by the promise of superior quality and precision in manufacturing, as well as economies of scale in mass-producing numerous standardised units, leading to enhanced productivity, rapid production, reduction of waste, and efficient allocation of resources.
1. Introduction
In a retrospective trajectory, the existence of the underlying principle behind off-site modular construction in the UK (i.e., relocation of on-site activities to a controlled factory environment) can be traced back to 1940s, and its use has been increasing ever since
[1]. Nevertheless, despite the long-standing interest in off-site modular construction, the UK has been lagging behind other developed nations in adopting this method. The reasons for this are frequently acknowledged to be deep-rooted and intertwined with the fragmented industry landscape
[2] in addition to comprehensive collaboration required due to the planning, coordination, installation, procurement, and scope of works
[1]. Additionally, the lethargic adoption of modular construction has been attributed to concerns surrounding costs, site productivity, and workmanship issues, which can all have a detrimental impact on the quality of the finished modular product during real-life implementation
[3]. Nevertheless, these implementation challenges appear to contradict early studies that promoted modular construction adoption, such as
[4][5][6], who posited that modular construction could simply reduce costs, boost productivity, and enhance quality control based on the input of experts’ opinions with a minimal attention towards practical/procedural aspects. Hence, it seems that reality of modular construction is considerably more multifaceted than initially thought. Anecdotally, Knightfrank’s report
[7] reveals that roughly 43% of respondents believe that modular construction will have no impact or even exacerbate the UK construction industry’s existing challenges without proper guidance for its real-life application.
Despite the widespread acknowledgment of the potential of off-site modular construction in addressing some of the UK’s most pressing construction issues, such as the housing crisis and the net-zero carbon objectives, the complete realisation of its perceived benefits requires a comprehensive understanding and resolution of the practical challenges encountered by contractors and other adopters in real-world situations. Accordingly, contractors and other adopters must anticipate and develop effective strategies to overcome such challenges, thereby instilling greater confidence in the viability of modular construction as a viable alternative for the UK construction industry.
2. Offsite Modular Construction in the UK
For over a century, the construction industry has relied heavily on conventional on-site and labour-intensive construction techniques, which have been deemed as the norm for the sector, including in the UK
[8]. Nevertheless, the world’s population is growing at an unprecedented pace, generating a higher demand for infrastructure and housing, which is further compounded by increasingly complex requirements such as sustainability and zero-carbon emissions
[9]. These changes require the construction industry to re-evaluate its traditional ways and adapt to the evolving landscape, as evident in the automobile and aviation sectors
[10]. Failure to respond effectively to these challenges may lead to the industry’s stagnation, which would ultimately result in the inability to meet the demands and expectations of the ever-changing society
[11]. In response, the construction industry has somewhat embraced modern construction techniques that are purportedly both efficient and sustainable, shifting the majority of on-site operations to factory-controlled environments in order to improve productivity
[12]. Although the definition of off-site construction (OSC) remains contentious, various studies have employed terms like modern methods of construction (MMC), modular construction (MiC), volumetric construction, industrialised construction, and prefabricated construction to refer to the practice of moving on-site work to a factory environment. However, regardless of terminology, the core principle remains the same: the relocation of on-site activities to a factory setting
[13][14].
Meanwhile, the UK CI serves as an intriguing case study to showcase the evolution of OSC and the integration of various terminologies to signify specific stages of this evolution over the last century. OSC’s origins can be traced back to the Roman occupation of Britain, with Lunt Fort’s archaeological site near Coventry built using components sourced from elsewhere
[1]. Joseph Paxton later revived this construction approach during the 1851 Great Exhibition, constructing the Crystal Palace in just nine months, and later dismantling it to be relocated elsewhere. After World War II, the UK deployed a prefab mission to meet the housing needs of the population; however, these prefabricated dwellings proved to be substandard in terms of durability and criteria, leading to negative public perceptions
[1][8]. To counter this perception, the UK CI introduced industrialised building concepts, with a focus on a closed construction process in a factory setting and leaving basic assembly activities for on-site work. This approach often employed large panel methods (i.e., panelised construction), and by the end of the 1970s, the UK CI had progressed towards volumetric construction, with modular construction becoming more prevalent in the 1990s
[13]. In 2019, the UK government defined modern methods of construction (MMC) as an umbrella term for seven categories of OSC forms that encompass all the previously listed taxonomies, all of which share the fundamental principle of relocating on-site work to a factory environment
[12].
The persistent promotion of MMC, particularly off-site modular construction in the UK, can be attributed to the perceived sustainability benefits derived from these practices. Evidently, the low level of embodied carbon during the manufacturing and assembly processes, as well as the potential for standardisation of designs and building components have been acknowledged by scholars and the UK government
[12][13][15]. By standardizing components, they can be more readily reused and recycled at the end of their lifecycle, thus reducing the future demand for raw materials and component production and representing a significant step towards achieving greater sustainability
[16].
Overall, it can be identified that the scholars have widely researched OSC in various forms and specifications. Despite this, there is unanimous agreement among scholars regarding the advantages and capabilities that OSC may provide over traditional construction, which will be discussed in the following section.
3. Advantages of Modular Construction
Table 1 summarises the key advantages of modular construction and the supporting literature. These advantages, ranging from improved quality control to enhanced mobility, make modular construction an attractive alternative to traditional construction methods for a wide range of building projects.
Table 1. Advantages of modular construction.
Although modular construction has been recognised for its numerous benefits, it is crucial to acknowledge the associated complexities and challenges, which will be discussed in the following section.
4. Challenges for Modular Construction
Table 2 presents a critical and advanced overview of the challenges faced by modular construction, organised into six broad categories. These challenges are based on previous research and are intended to provide a comprehensive understanding of the challenges faced by the industry.
Table 2. Challenges for modular construction.
After extensively reviewing the literature, it is apparent that modular construction faces a plethora of challenges, primarily pertaining to design, logistics, and financial overruns. However, there exists a paucity of research delving into the reasons behind its underutilisation and the persistent preference for traditional construction techniques.