## 1 Introduction  
To bridge the gap between the capacity of housing supply and the explosion of housing demand in connection to rapid urbanisation, the construction industry has been seeking innovative materials and technologies that can provide more high-quality housing using less construction time. Within this context, off-site construction has been acknowledged worldwide to play a key role in providing housing with enhanced quality, productivity, safety and efficiency [1; 2; 3]. With the development of industrialisation, off-site construction has been embraced by the construction industry in an array of countries and regions, involving the United Kingdom, the United States, Japan, Sweden, Germany, Australia, Singapore, Malaysia, mainland China, and Hong Kong [4; 5; 6; 7; 8; 9].  
Following the level of prefabrication adoption, [PERSON] [10] divided off-site construction into four categories, in terms of \"Component manufacture and sub-assembly\", \"Non-volumetric pre-assembly\", \"Volumetric pre-assembly\" and \"Modular Building\". Along with the increasing degree of completeness, more components of the construction are produced in a factory, forming non-volumetric and volumetric (three-dimensional) modules. Peaking the highest level of prefabrication, modular buildings are identified as a state-of-the-art technology reshaping the construction industry [11]. Modular buildings endeavour to produce maximum prefabricated modules in off-site manufacturing factories, whilst minimising on-site construction activities. Volumetric pre-assembly modules have been installed with finished floors, walls, ceilings, cabinets as well as mechanical, electrical and plumbing (MEP) services, in advance of transporting from domestic or overseas factories to the construction site where the modules are assembled [12; 13].  
In recent decades, modular buildings have been implemented in both public and private sectors, providing an eligible solution for buildings composed of repetitive units, for instance, residential buildings, hotels, student accommodations and hospital wards [14; 15; 16; 17; 18]. Previous works and projects demonstrated the strengths of modular buildings involving reduced construction time, diminished materials waste, improved quality, decreased labour demand and safe work environment [5; 19; 20; 21; 22; 23]. Considering the great underlying potential of modular buildings, the industry coupled with academics and endeavoured to propose varied solutions involving building information modelling (BIM), and design for manufacture and assembly (DfMA) facilitating the application of modular buildings [24; 25; 26; 27; 28; 29]. However, in comparison to conventional buildings, the application of modular buildings is still relatively low, especially for high-rise buildings. For instance, off-site construction in Australia accounts for merely 3 percent of the total construction industry's output, of which most adoptions are precast components and panelised walls [9]. The most uptake of modular construction is modular houses and low-rise buildings, by contrast, high-rise modular buildings are emerging with the slow pace of development [11].  
This paper aims to identify the constraints of high-rise modular buildings to promote its application as well as exploiting the potential market. A comprehensive literature review is firstly conducted to list preliminary constraints. Following that, a focus group study is carried out to extract crucial factors forming questionnaires for all stakeholders of modular construction. Based on the responses of the questionnaires, constraints of high-rise modular buildings are further analysed and discussed in association with an empirical case study. This research will contribute to getting the whole picture of modular construction and propose valuable solutions to the dominant constraints.