Key influences to cost modelling and analysis in off-site construction: research trends and GAP analysis

Figures & data

Figure 1. Further combinations of the relevant criteria.

Figure 2. Inclusion and exclusion criteria for the research.

Table 1. Summary of key costing in OSC.

Cost	Description	Authors
Capital and Development	Capital or development costs represent the total costs required for a project to become operational (Mao et al., 2016). Capital costs are therefore typically those for outlays of associated works to a project’s design and construction. In OSC systems, these can relate to elements associated with both ‘On-Site and Off-Site’ processes (e.g. associated resource-based costs for materials, plant and machinery). They will therefore represent enabling works for design, manufacturing and prefabrication, distribution, and On-Site installation of prefabricated components and modules. Extensive capital costs are normally needed for OSC systems to develop the end-to-end production system between design and implementation that have to interface between Off-Site and On-Site planning and management including to required processes and systems (Mao et al., 2016).	Mao et al. (2016)
Site Establishment	Site establishment costs are essential for OSC, thus forming a vital part of the contractor’s preliminaries. However, they are in the main excluded in the prefabricated buildings and building unit’s cost centre within the NRM1 while playing a significant element with OSC systems. The costs generally relate to the main contractor’s and common user temporary site accommodation e.g. offices, conference/meeting rooms, canteens, kitchens, drying rooms and washrooms (Obi et al., 2021). The distribution of such costs in OSC systems can depend on the level of ‘On-Site and Off-Site’ activities and processes. Therefore, their thorough understanding can be critical to successful project cost modelling.
On-Site Construction	OSC systems can decrease On-Site construction costs (Smith, Hammeran, & Lockwood, 2012; Lee & Kim, 2017). On-Site construction costs in OSC systems can include elements relating to site preparation, demolition and clearance, including any appropriate and necessary excavations (Salama, Salah, & Moselhi, 2017). Excavations can also relate to the substructure’s enabling works such as for foundations, pilling and any filling. Costs for On-Site construction will cover elements of any slabs and accompanying drainage. Other costs within this cost centre relate to external and Off-Site works necessary to enable the installation of the modular modules or prefabrication systems. Additionally, costs will cover roads and footpaths, supporting signage and external streetlights, associate landscaping and fencing, and parking for vehicles or cycles, including related waste recycling and storage spaces. Depending on the project’s nature, special works for example a pumping station and traffic management may need accounting for.	Smith et al. (2012); Lee and Kim (2017); Salama, Moselhi, and Al-Hussein (2021); Sutrisna et al. (2019)
Modular Manufacturing	In a factory, materials that would otherwise be delivered and processed On-Site feed into the manufacture of modules within factory controlled conditions (Lawrence and Morton, 1993). The modules are then transported to the factory through an appropriate logistics programme (Arashpour, Wakefield, Blismas, & Maqsood, 2015). Palpable benefits of modular construction include opportunities in processes and quality optimisation to augment cost performance (Lawrence and Morton, 1993). The factory process of modular manufacturing ensures simultaneous site preparation and construction processes as well as controlled and optimised module manufacturing (Sutrisna & Goulding, 2019). However, productivity issues have been reported in modular manufacturing processes resulting from specific uncertainties that will ultimately impact cost (Vigneault, Boton, Chong, & Cooper-Cooke, 2020). In modular manufacturing, it is essential that significant costs are separated during cost modelling, for example complete prefabricated building or type of prefabricated room unit (stating the number of units) according to NRM1 based on GIFA. Therefore, the cost will cover complete self-finished prefabricated modular building systems or units included as part of building systems.	Lawrence and Morton (1993); Vigneault et al. (2020); (Arashpour et al., 2015); Sutrisna et al. (2019)
Factory Related Preliminaries	Cost modelling in OSC systems has a significant element that is factory based. In the factory, processes can optimise task efficiency while modelling, reducing task complexity that can ultimately reduce resource-related costs (Haas, O’Connor, & Tucker, 2000). This means that unlike conventional construction processes, front-loading of costs significantly weighs towards factory processes (Mao et al., 2016). The front-loading of costs in these processes now requires new techniques for cost optimisation, perhaps on the basis seen in sectors for example in manufacturing. More recently, there has been a proliferation of techniques to optimise factory costs that ultimately reflect in factory preliminaries. There are now many methods to optimise factory processes, including mathematical modelling for supporting factory process planning or facilitating multiple resource environments (Lee & Kim, 2017). Heuristics-based techniques have also been proposed to calculate the parameters of processes requiring minimum costs (Lawrence and Morton, 1993).	Lee and Kim (2017); Haas et al. (2000); Lawrence and Morton (1993); Mao et al. (2016)
Project Team Design Fees	There will be a host of project teams and design fees dependent on the scale and specifics of the project. Such costs relate to legal and regulatory activities, local and national authority charges, planning consultation, planning permission, discharge of planning conditions applications and site investigations. It may suffice such costs are accounted for among many other general costs, for example, the design team. Preconstruction remediations may be required following any investigations for example ecological/habitat and topographical compliance, tree protection noise and dust pollution, asbestos, any legacy contaminations from previous land use, and logistics support for necessary plant and machinery (Edwards et al., 2003). These costs will be on top of the design fees for the wider design team, including architects, structure and civil engineers, M&E engineers, and specialised design teams e.g. for acoustics. Other costs in this cost centre will relate to building control and warranties and energy performance certification that is increasingly integral to new projects considering a move towards improved environmental performance of new builds. Searches may be required for activities, including legacy activities and utility suppliers, to support design and planning decision making. Lastly, in this cost centre, costs for insurance both for latent defects and site operations must be accounted for.	Edwards et al., (2003)
Contractor’s Overheads	There is a significant cost for overheads in OSC systems, mainly in the early stages of operational and production setup. The cost distribution will be heavier towards offsite construction activities. Factory setup costs will therefore be a significant cost element in OSC systems. These will cover factors related to factory security, staff and necessary accommodation for example office space and associated and supporting facilities and services (e.g. containers for storage and transportation). Costs will also relate to any temporary supporting works for instance factory-based showcasing together with associated services and supporting works.
Abnormal Costs	According to the NRM 3, examples of abnormal costs are those relating to issues for example: ‘access constraints, legacy data issues, unforeseen events due to the nature of the assessment of works, statutory bodies and listed buildings’. Abnormal costs are therefore considered to be site-specific in OSC and can include legacy issues for instance relating to asbestos clearance on brown field sites. These poor ground conditions impact on foundations or site-specific investigations for contamination and can significantly influence the general cost modelling of a project, translating into unusually higher group elemental and overall costs. While solutions such as site benchmarking are recommended to address abnormal costs (RMB-Council, 2013), being site-specific means that these will be continually influencing.	RMB-Council (2013)

Figure 3. Yearly distribution of articles.

Figure 4. Distribution of articles in journals.

Figure 5. Link between cost modelling and emergent concepts in OSC.

Table 2. Emergent keywords.

Keyword	No. of occurrences	Average pub. year
Prefabrication	37	2018
Cost analysis	22	2017
Economic and social effects	21	2017
OSC/ Modular Construction	272	2017
Costs /Cost Estimation	135	2013
Cost model/ Cost Modelling	43	2016
Life cycle	47	2016
Cost benefit analysis	122	2015
Construction costs	22	2014
Project management	90	2013
Cost modelling	79	2008

Figure 6. Connection between key authors.

Figure 7. Comparison of cost influences from research.

Table 3. Costing elements covered in research.

Costing element	Author
Life Cycle Costs	Pan and Sidwell (2011), Yunus and Yang (2012), Nahmens and Ikuma (2012), Li et al. (2014), Mao et al. (2016), Kamali and Hewage (2016), Kamali and Hewage (2017), Hong, Shen, Li, Zhang, and Zhang (2018), Dong et al. (2018), Kamali et al. (2019), Daget and Zhang (2019), Hsu et al. (2019), Balasbaneh and Ramli (2020), Heesbeen and Prieto (2020), Akbarieh et al. (2020), Jiang, Huang, and Peng (2020), Gbadamosi et al. (2020), Turner et al. (2021), Kedir and Hall (2021)
Preliminary Costs	Pan and Sidwell (2011), Mao et al. (2016), Hong et al. (2018)
Factory related Preliminaries	–
Capital/Development Costs	Pan and Sidwell (2011), Zhang and Skitmore (2012), Mao et al. (2016), Hong et al. (2018), Jiang et al. (2020), Gusmao Brissi, Debs, and Elwakil (2021), Vaz-Serra, Wasim, and Egglestone (2021)
Prefabrication Costs	Jonsson and Rudberg (2014), Shahtaheri et al. (2017), Wang et al. (2018), Sutrisna et al. (2019), Hsu et al. (2019), Jiang et al. (2020)
Site establishment costs	Mao et al. (2016)
Project team design fees	Mao et al. (2016)
Contractor’s Overheads	Pan and Sidwell (2011), Mao et al. (2016)
Onsite Construction Costs	Mao et al. (2016)

Figure 8. Elemental costs influences.

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