Key points
- Volumetric modular: 3D units made in the factory with floor, walls and ceiling, plus part of the finishes and services.
- Panelised: flat elements (walls, floors, roofs) made in the factory and assembled on site.
- McKinsey (2019): modular can speed up projects by 20–50% and save up to 20% of cost, given the right scale, repetition and design.
- Permanent modular buildings are designed to meet the same building codes as conventional construction.
Written for:OwnerContractorSupplier
"Prefab" is a broad term covering anything made in a factory and then transported and installed on site. The idea: move a large part of the work from a weather-exposed site to a controlled factory environment.
1) The main types
| Type | Description |
|---|---|
| Volumetric modular | Units each with floor, walls and ceiling, usually including structural elements and part of the finishes and electrical, mechanical and plumbing services1. |
| Panelised | Flat elements (mostly wall panels, sometimes floors or roofs) made in the factory, consisting of the structural elements only (framing and sheathing)1. |
| Precast concrete | Concrete elements (walls, slabs, columns) cast in the factory and installed by crane. |
| Light steel frames | Light galvanised steel sections formed and assembled as a structure for walls and roofs. See steel framing. |
Permanent or relocatable?
The Modular Building Institute (MBI) distinguishes between2:
- Permanent modular construction (PMC): designed to stay in one place for the long term, with a life comparable to a conventional building.
- Relocatable buildings: designed for shorter periods and repeated moves between sites.
In both cases the units use the same designs and materials as conventional construction and meet the same codes and standards2.
2) The benefits in numbers
- Speed: McKinsey found modular construction can deliver projects 20% to 50% faster than traditional methods, with potential savings of up to 20% in cost34.
- Parallel work: units are manufactured at the same time as site preparation and foundations1.
- Quality and safety: the factory environment allows continuous monitoring and reduces weather damage and site hazards1.
- Less waste: the US wood products council notes waste in modular manufacturing is typically under 5%1.
3) Challenges and conditions for success
- McKinsey stresses that realising the benefits requires choosing the right materials and design, overcoming design, manufacturing, technology, logistics and assembly challenges, and working where there is scale and repetition3.
- It warns of a risk of losses of up to 10% if transport or material costs rise3.
- Transport: volumetric units are limited by road and truck dimensions and need cranes and suitable site access.
- Late changes: changing the design after manufacturing starts is expensive; decisions must be made early.
4) When is prefab a smart choice?
- Repetitive projects: housing complexes, worker accommodation, classrooms, hotels with similar units.
- Remote sites or difficult working conditions.
- When time is decisive.
- Home additions: an extra room, an outdoor annex, a garden office (subject to municipal requirements).
5) Checklist before buying a prefab building
- Permits: check municipal requirements for the building type and location before buying.
- Code compliance: ask for proof the design complies with the Saudi Building Code (structural, fire, electrical, plumbing, energy).
- Thermal insulation: ask for the wall and roof U-values, which matter a lot in lightweight buildings. See thermal insulation.
- Fire resistance: what are the wall and insulation materials and their fire classification? See sandwich panels.
- Foundations: who designs and builds them, and does it include a soil investigation?
- Corrosion and moisture protection: especially for steel structures and coastal sites.
- Transport and installation: does the price include transport, crane and connections?
- Warranty and maintenance: warranty periods for the structure, leaks and finishes.
Sources
The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.
- 1Modular Construction (volumetric vs panelized) — WoodWorks — Wood Products Council
- 2Relocatable Buildings vs. Permanent Modular Construction: What's the Difference? — Modular Building Institute (MBI)
- 3Modular can deliver projects 50% faster, McKinsey finds — Construction Dive
- 4Modular construction: From projects to products — McKinsey & Company
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This guide is for general information and planning, based on standards and technical guides. Standards are revised periodically; in Saudi Arabia the binding reference is the Saudi Building Code and the instructions of the competent authorities. For design and execution, use a licensed engineer or technician. Found an error? Tell us.