What is OSM?

Offsite manufacturing (OSM) and modular construction are methods of building where components are made remotely, then delivered and assembled on site. Components may include wall, roof or floor panels or even complete rooms.

Whilst historically these construction methods have been used widely on larger scale projects, OSM processes and technologies have now developed and are becoming more cost-effective, therefore being suitable for use on smaller developments and even single unit residential schemes.

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OSM can be particularly effective on constrained London sites, where there are often significant limitations on working area and storage. Work can also be carried out in the factory at the same time that foundations and substructures are being prepared on site, leading to time efficiencies. Disruption to neighbours is therefore minimised, this is a key benefit on dense urban sites in London.

The improvements in construction speed and potential cost-effectiveness of OSM may be a solution to delivering housing in London more rapidly.


A fast construction process also typically leads to reduced preliminaries costs, for example in scaffolding hire. Whilst the programme is shorter, it should also be more reliable, as factory based work is not weather dependent.

OSM can also be closely controlled, meaning the work may be completed to a higher standard of quality and with better safety standards. This also gives the potential for more energy efficient buildings, with improved air tightness due to precision fitted seals and membranes. There is a reduction in waste during construction and there are also building life-cycle considerations.

Modular construction may even allow for re-use and re-location of the structure in the future.



Depending on the materials and systems chosen, OSM may impose design limitations. For example, CLT has only been used for buildings up to 9 storeys in the UK. It is also crucial to reach an early design freeze as flexibility for changes on site is limited.

Tolerances need to be considered and built-in early on in the design process, and this is particularly important when integrating OSM with existing buildings.


Currently there is a lack of UK suppliers for larger prefabricated components, when compared with the rest of Europe. This can lead to comparatively higher costs for OSM projects in the UK. There is also a lack of public awareness of these modern methods of construction which may lead to a reluctance to use them.


A careful analysis of cost, quality and time will determine the direction of any building project. The potential for shorter construction programmes and an improved quality of product are key selling points for OSM. Whilst we are already seeing the rise of modular construction in London, we are likely to see a more dramatic uptake in the future, as cost-effectiveness also becomes a key selling point, arising from supply chain improvements and industry familiarity with these technologies.

Current AFL projects can be viewed on our housing page, or contact us to discuss a project that may benefit from offsite manufacturing.

Image credits

Architect: Jørn Utzon, prefabricated Bagsvaerd Church, Copenhagen.

Photographer: SEIER+SEIER.