Prefabrication moves risk from the site to the factory

What follows is the working version of offsite construction: the decisions in the order you actually meet them, with the reasoning attached.

The trade is variability for dependency

Site construction is exposed to weather, coordination between trades and the availability of skilled labour on any given week. Factory production removes most of that variability by working indoors with a fixed workforce and a repeatable sequence. What it introduces is dependency on a single supplier, since components designed for one factory cannot usually be made by another.

If the factory fails financially or falls behind, the project has no alternative source and the site stands idle waiting for parts. This is a different risk rather than a smaller one, and it sits with the developer while looking like a supplier problem.

The saving is in programme, not materials

Prefabricated components rarely cost less than site-built equivalents once transport, craneage and the factory overhead are included. The gain appears in the construction programme, because foundations and superstructure can proceed while units are being manufactured in parallel.

At street level, a shorter programme reduces finance costs, brings rental or sales income forward and cuts the period of disruption for neighbours. On a constrained urban site the reduction in deliveries, scaffolding and noise can be the decisive advantage rather than the money. This means prefabrication makes most sense where finance costs are high or site access is genuinely difficult, not everywhere.

Repetition is the condition for the benefit

Factory economics depend on making the same thing many times, since each variation requires re-tooling, re-drawing and separate approval. Housing schemes that vary unit types heavily to satisfy planning requirements defeat the repetition the method needs.

Where a client builds continuously to a standard design, the learning accumulates and the cost falls with each successive scheme. One-off adoption by a developer with no follow-on pipeline captures none of that learning and pays the full set-up cost. This is why the method has succeeded most consistently for institutional clients with rolling programmes rather than for speculative one-off projects.

Design decisions move much earlier

A factory needs final drawings before manufacture starts, so decisions that on a traditional site could be deferred must be settled at the outset. That is a discipline with real benefits, since late changes are the main source of cost growth in conventional construction.

At street level, it is also unforgiving, because a change requested after production has begun can be more expensive than the same change on site. Clients accustomed to adjusting details during construction find the process constraining, and the friction has ended several programmes.

The method rewards organisations that can make decisions early and hold them, which is a management capability rather than a technical one.

Durability and repair are the open questions

Sealed factory-made joints, concealed connections and proprietary systems can be difficult to inspect and to repair decades later. If the manufacturer no longer exists, replacement components may not be obtainable and repairs become bespoke and expensive. Some historic system-built housing performed poorly for exactly these reasons, and that history still shapes lender and insurer caution.

Measured properly, modern systems are not the same as those, and the general lesson about long-term parts availability has not gone away. Buyers and housing providers are reasonable to ask how a component will be replaced in thirty years, and answers are often thin.

Local statute governs most of this, and it varies street by street.

Where it clearly works

Bathroom and kitchen pods, plant modules and prefabricated facade panels are widely used and rarely controversial, since they are components rather than whole buildings. Volumetric construction of whole rooms suits repetitive uses such as student accommodation, hotels and single-aspect apartment blocks. Structural systems in timber or light steel sit between the two and offer speed without committing the whole building to one supplier.

Over a decade, the pragmatic position is to prefabricate the repetitive parts and build the site-specific parts conventionally, which most successful projects do. Presenting the choice as factory against site misses that almost every modern building is already a mixture of both.

The takeaway

Ask where the risk went. Prefabrication moves it; it does not remove it.

Cities are built by a thousand small permissions, not one big plan.