Modular construction aims to deliver projects more efficiently, with improved quality and a lower environmental impact. Its success, however, depends on careful specification at every stage: design, manufacture, transport, installation, and operation. This article outlines a framework for good practice for the specification of modular construction projects, encompassing design integration, performance standards, compliance, digital processes, and lifecycle considerations.
By using Uniclass classification and writing system‑based specifications in NBS Chorus, the performance and evidence of compliance requirements can be stated and help to maintain the golden thread of information under the ISO 19650 series.
Modern Methods of Construction (MMC) use offsite prefabrication techniques, including modular volumetric systems (whole units constructed offsite), panelized assemblies (a series of elements and components, put together on site) and ‘additive’ manufacturing (use of 3D printed components for potentially complex geometries). Components or even entire structures are built in controlled factory environments and transported to the site for assembly. To achieve the full benefits, specifications must be robust, covering design, performance, safety, and regulatory standards.
The benefits of using modular products, however, rely on having clear, precise specifications which help to manage risk and ensure compliance with performance, health and safety, and regulatory requirements throughout the project.
Classifying systems and products using Uniclass supports coordination with building information management (BIM) and facilitates asset‑level data handover.
The following is an example of NBS Chorus relevant Uniclass content:
Pr_20_65_50 Modular products (Product table → Structural and general products→ Prefabricated buildings and structures).
Use this code for the principal modular building products in models, schedules, and specifications.
Figure 1: Pr_20_65_95 Volumetric modules
Design considerations
Designing for modular construction requires applying modular principles from the start of the project and using design for manufacture and assembly (DfMA principles (the grid, module sizes, lifting points, logistics). The design will need to consider some specific aspects of the construction, for example, the load paths, progressive collapse, fire compartmentation, and the serviceability of the building from factory to in‑use. The building performance is also a critical area of design; acoustic and thermal performance, airtightness, and accessibility should be addressed without compromising the integrity of fire or thermal performance systems.
Safety
Safety considerations cover such aspects as fire, structural, and installation, fire stopping of compartment, cavity barriers installed where required (including hidden voids that can be created when modular units are stacked or abut one another), and intumescent seals. Fire concerns include the lack of fire performance certificates or lack of clarity over fire structural design, as highlighted in guidance issued by the Royal Institution of Chartered Surveyors (RICS). Structural safety requires verification of load-bearing capacity and stability during lifting and installation. Safe installation involves the need for risk assessments for crane operations, working at height, and temporary work. All safety measures must comply with the Building Safety Act and CDM Regulations.
Specifications are more than just technical documents. A robust specification removes ambiguity, aligns with Building Safety Act duties, satisfies funder/insurer expectations, and supports warranty acceptance. Clear Uniclass coding and NBS Chorus information structures improve coordination across drawings, schedules, and model data (such as BIM, etc.) and can support gateway evidence and the golden thread. The specification is an important document that can be evidence of the requirements and intent at specific points in the construction process and is often used as an important or critical reference document in matters of contractual clarity, disagreements, and litigation.
1. Clarity and precision
2. Compliance with standards
In the UK, all building work, including modular, must comply with the UK Building Regulations. All modular products must comply with relevant standards covering fire safety, acoustics, structural integrity, and thermal and energy performance. Specifications should reference the appropriate British Standards (BS), ISO standards, and MMC frameworks.
3. Lifecycle and sustainability
A robust specification should embed clear sustainability and lifecycle considerations to ensure long-term performance and support organizational net-zero ambitions. Key elements include:
4. Digital integration
Digital coordination is essential for successful MMC delivery. Best practice involves embedding specifications within BIM workflows, using digital product passports to ensure traceability, and making sure modular components are fully interoperable with digital systems. Digital Product Passports are moving toward mandatory status in the EU, with construction products included as part of the revised CPR 2024. While not yet required within the UK domestic market, UK manufacturers supplying the EU must comply. Adoption in the UK is expected to grow due to regulatory alignment and market demand.
5. Flexibility and modularity
Specifications should embed controlled flexibility that enables variation where needed while safeguarding core performance and compliance requirements. Clear rules for configuration, compatibility, and interface standards must be established to ensure components integrate reliably across different modular arrangements. Solutions should be designed to be scalable, repeatable, and efficient to assemble, supporting streamlined delivery and consistent quality across projects. At the same time, specifications must allow for future adaptability - such as reconfiguration, extension, or replacement without compromising safety, regulatory obligations, or long‑term asset performance.
Site safety during the installation of modular units is critical to preventing accidents and ensuring full compliance with CDM Regulations and HSE guidance. Specifications should set out clear requirements for safe installation practices, including:
By embedding these safety requirements into the specification, projects can reduce risk, maintain compliance, and ensure a safe, controlled installation environment for all site personnel.
Sustainability is a core objective of modular construction, with a strong focus on reducing waste, optimizing material use, and lowering carbon emissions throughout the project lifecycle. Modular units can be designed for ease of disassembly, enabling components to be reused, reconfigured, or recycled at end‑of‑life, thereby supporting circular economy principles and extending the value of built assets.
Specifications play a vital role in driving these outcomes by setting clear targets for waste reduction, recycled content, embodied carbon performance, and airtightness. They can also mandate sustainable material choices, for example, responsibly sourced timber, low‑carbon concrete alternatives, or recyclable metals manufactured under robust factory‑controlled quality systems such as ISO 9001, ISO 14001, and ISO 45001, where relevant.
Performance requirements may also be embedded within the Preliminaries section of the specification to ensure high levels of airtightness, effective thermal insulation, and accurate embodied carbon reporting in accordance with standards such as BS ISO 14068-1. By integrating these sustainability measures from the outset, modular projects can achieve more consistent environmental performance, contribute to net‑zero goals, and deliver long‑term value for clients and end users.
1. Fire
Fire performance is a critical consideration when specifying modular construction products, particularly within Modern Methods of Construction (MMC). Because modular buildings are manufactured off-site and assembled on-site, fire safety must be integrated into product design, factory processes, and installation from the start to ensure the integrity of the fire performance is maintained.
Fire performance includes both fire resistance and reaction-to-fire properties. In MMC systems, factory-controlled installation of fire barriers and intumescent seals can improve consistency and reduce workmanship variability. However, fire-stopping and compartmentation may be compromised during transport and on-site assembly, particularly at module interfaces where unintended voids or gaps can occur. Concealed junctions may also restrict post-installation inspection. Insurer and fire sector guidance, including that from the Association of British Insurers and the National Fire Chiefs Council, emphasizes the need for robust quality assurance and inspection regimes to address these risks.
Specify products clearly: For example, use the following Uniclass product codes for fire integrity/compartmentation items:
2. Structural
Structural integrity is essential in modular construction to ensure safe lifting, transport, and long-term performance. Modules engineered for strength and precision help reduce structural defects, and off-site quality assurance supports compliance with Eurocodes. However, dynamic loads during lifting and transport can cause damage if not responsibly managed, and incorrect stacking or inadequate temporary works can compromise stability.
Where structural insulated panels (SIPs) are used, specify using e.g.
3. Durability
Durability in modular construction depends on careful material selection, detailing, installation, and maintenance. Factory-controlled assembly reduces exposure to moisture, improving longevity, and the use of engineered materials can enhance overall durability.
4. Acoustic
Acoustic performance in modular construction covers both sound insulation and vibration. Precision manufacturing improves airtightness and reduces flanking paths, while acoustic insulation can also be integrated during factory assembly. However, lightweight modules can transmit impact noise, so resilient layers and careful detailing are important. Poorly detailed junctions and connection details of modular units need to be considered to prevent unintentional acoustic bridging.
5. Thermal and airtightness
Thermal performance in modular construction must comply with the Building Regulations and guidance and address the risk of overheating. High levels of airtightness are achievable where modules are built under factory conditions, which improves energy efficiency. The use of more advanced technologies and high performing insulation and use of thermal breaks can be incorporated to achieve high levels of thermal performance. However, poor detailing at junctions can create thermal bridges, and lightweight structures may be prone to overheating and require passive cooling strategies.
6. Compliance and standards
It is important that modular construction follows the same Building Regulations requirements as other construction methods within the UK, although meeting these requirements may be difficult and require specific discussion with Building Control authorities around the associated risks. This can also lead to insurers being concerned about this type of construction.
Although there are currently no specific standards covering MMC, The International Standards Organization (ISO) has established a new sub committee to develop global standards for prefabricated buildings, which when available, will be adopted in the UK as BS ISO standards.
Modular construction products must follow the legislative requirements as products and carry UKCA marking for Great Britain and UKNI marking for Northern Ireland and comply with the Construction Products Regulation. Early engagement with insurers is recommended to ensure alignment with property protection standards, and third-party certifications such as BBA or BRE Global should be verified.
Within the UK, BSI has published PAS 8700 to strengthen confidence in Modern Methods of Construction (MMC) for residential developments. This is the first comprehensive framework for consistent and safe adoption of MMC across new-built housing projects, whether partially or fully using offsite construction. PAS 8700 sets requirements for applying Design for Manufacture and Assembly (DfMA) and MMC throughout the project lifecycle; from early design and factory production to onsite assembly, maintenance, adaptation, and end-of-life (reuse). There are also useful references in the NHBC Technical Standards | NHBC Part 11, which has been added for MMC and includes site assembly, maintenance, adaptation, and end-of-life.
Key standards commonly referenced in modular specifications
A robust modular construction specification should reference the same relevant national and international standards as all construction methods, that guide quality, safety, and information management. Commonly cited standards include:
Structural design standards
MMC consideration: Load cases must include manufacture, transport, lifting, and installation phases.
Acoustic standards
Thermal, airtightness, and energy standards
Manufacturing, quality, and assurance
Management systems
MMC-specific assurance
Digital information and BIM
Other relevant standards and PAS documents
The sequence of deliveries needs to match the installation. Fire The integrity of maintaining fire performance at junctions and service penetrations is critical. Ensure inspections are carried out and installation is verified, particularly for thermal breaks and cavity barriers. Use of accredited installers should be undertaken for critical connections.
The delivery and installation of modular construction units are critical stages in ensuring quality, safety, and performance in Modern Methods of Construction (MMC). Effective planning, coordination, and verification are essential to maintain the integrity of factory‑built modules and achieve efficient on‑site assembly.
Delivery
Important aspects to enable a successful delivery.
Installation
The following aspects are important considerations for successful installation:
Well planned delivery and precise installation are essential to preserve the quality achieved in the factory, reduce on‑site risks, and ensure modular buildings perform as intended throughout their lifecycle.
Ensuring that a project meets its design intent and performance requirements is essential, particularly as the construction sector places greater emphasis on accountability and transparent information management. Collecting evidence during the post‑construction phase plays a vital role in demonstrating compliance and maintaining a reliable digital golden thread of information in line with the BS EN ISO 19650 series.
Key documentation, such as Declarations of Performance (DoPs), fire safety certification, and mock‑up installations, helps verify that systems and components perform as intended. These measures also enable meaningful comparisons between factory‑controlled production and on‑site installation quality. Where additional assurance is required, accredited third‑party certification can provide independent verification and reinforce confidence in the finished solution.
Design and compliance
Important considerations for the design of modular and MMC based projects include:
Factory production control
Key aspects to consider during factory‑based manufacturing include:
Testing and certification
During the testing and certification phase, project teams should prioritize:
On site verification
On-site assurance remains a critical part of the process:
Ongoing assurance
To maintain long-term confidence in the completed project:
Modular construction can only achieve its full potential for speed, quality, and sustainability when specifications are clear, testable, and digitally managed. Embedding modular principles from the outset, defining system level performance, and rigorously controlling interfaces, installation, and verification all help to reduce risk and ensure regulatory compliance.
Aligning specifications with PAS 8700:2025, UK Building Regulations, Uniclass classification, NBS Chorus authoring, and ISO 19650 information management creates a consistent, collaborative framework. This approach strengthens approvals and warranty acceptance (such as NHBC Standards 2025 – Part 11 (MMC).), improves quality and consistency, supports innovation, and ultimately delivers safer, more efficient, and more sustainable modular buildings.