Planned revisions to Approved Document B (ADB) could bring the UK’s mass timber construction sector almost to a halt, according to Buro Happold’s Matthew Duckett.
On 25 March, the government published proposed amendments to ADB, he statutory fire safety guidance for new buildings in England, which, if implemented, could significantly restrict the use of mass timber in buildings taller than two storeys.
Duckett argues that the proposals risk reinforcing misconceptions about timber construction at a time when the industry is already facing challenges around perception. More significantly, he contends that the measures lack a robust technical basis, would be difficult to implement commercially, and conflict with wider government objectives for housing delivery and decarbonisation.
The consultation, which remains open until 1 July, proposes that any building with an occupiable floor located more than 11 metres above ground level must use loadbearing structural elements achieving a minimum reaction-to-fire classification of A2-s3, d2. Timber is unable to meet this classification requirement.
If adopted, the changes would effectively extend restrictions currently associated with higher-risk buildings to a much broader range of low- and mid-rise developments. The proposals make no distinction between different occupancy types or levels of risk, instead applying requirements based largely on the structural material used.
As a result, structural timber could be excluded from many mid-rise developments, including schools, housing schemes, rooftop extensions, offices and laboratory buildings, sectors where timber construction has been gaining momentum as a low-carbon alternative.
Additional measures are proposed for buildings between 4.5m and 11m in height. These would further limit the use of exposed combustible materials, potentially reducing many of the architectural and biophilic benefits associated with timber buildings while requiring additional fire-protection finishes and detailing.
Concerns over the 11m threshold
ADB already recognises timber as a less common construction material and, following changes introduced in 2022, combustible materials were banned from external walls in certain building types. The guidance also notes that timber may not be appropriate for larger or taller buildings above 18 metres.
According to Duckett, these earlier changes contributed to greater caution among clients, insurers and approval bodies, leading many timber projects to be replaced with steel or concrete alternatives as designs progressed.
The government has cited research published by BRE Global in September 2025 as justification for lowering the threshold from 18m to 11m. However, Duckett argues that the report does not provide a clear rationale for abandoning the existing limit. He notes that some of the evidence referenced relates to observations of post-fire smouldering from a single cross-laminated timber compartment test that was not specifically designed to assess smouldering behaviour.
He also questions the relevance of smouldering occurring hours after a fire in buildings that would typically be evacuated within minutes under modern fire safety standards. In his view, the proposals fail to differentiate adequately between building uses and associated levels of risk.
The current 18m threshold aligns with several existing regulatory frameworks, including the definition of a Higher-Risk Building under the Building Safety Act, as well as additional fire-fighting and evacuation requirements contained within BS9999. Duckett argues there is no transparent evidence-based justification for lowering the limit and warns that, once introduced into regulation, such a change would be extremely difficult to reverse.
The challenge of alternative compliance routes
While ADB is guidance rather than legislation, and alternative compliance pathways technically remain available, Duckett believes this offers little reassurance in practice.
Building control bodies, insurers, lenders and warranty providers often regard compliance with ADB as the benchmark against which schemes are assessed. Although alternative approaches can be accepted where they demonstrate an equivalent level of safety, the proposed guidance would effectively define safety through the use of non-combustible structural materials.
Duckett argues this creates a significant obstacle for timber projects. Performance-based fire engineering solutions may struggle to satisfy a framework that treats non-combustibility as the primary measure of safety.
He also points to the Building Safety Regulator’s competency requirements, suggesting that many Registered Building Control Approvers may not have the specialist expertise required to assess bespoke fire engineering solutions for otherwise conventional mid-rise timber buildings.
The likely consequence, he says, is that clients will be discouraged from pursuing timber schemes, funders will become more cautious, and projects will revert to steel and concrete construction.
Clash with government ambitions
Duckett highlights what he sees as a contradiction between the proposed changes and the government’s Timber in Construction Roadmap.
The roadmap identifies timber as a key material in delivering the government’s target of 1.5 million homes during the current Parliament, while also helping to reduce embodied carbon in buildings and support a forestry and timber processing sector that contributes significantly to the UK economy.
It specifically identifies mid-rise buildings as one of the most promising opportunities for large-scale adoption of mass timber.
According to Duckett, the proposed ADB revisions directly undermine these ambitions. Developers and housing providers unwilling or unable to absorb the additional costs and complexity of bespoke fire engineering are likely to revert to traditional construction materials, while the consultation’s impact assessment gives limited consideration to the resulting carbon implications or effects on the forestry sector.
A different approach
Rather than introducing stricter material-based restrictions, Duckett argues that any thresholds should be proportionate, evidence-led and clearly justified. He advocates greater investment in mechanisms that could streamline approval processes for timber buildings, including the development of pre-approved design details and standardised compliance pathways.
Maintaining the existing 18m threshold, while expanding pre-assessed compliance routes for timber structures below that height, would provide certainty for clients and regulators while enabling innovation to continue.
Duckett points to initiatives such as the New Model Building programme, a pre-assessed design methodology for multi-storey mass timber housing developed by Waugh Thistleton, Buro Happold and fire engineers at UCL, as examples of the type of collaborative, evidence-based work that could help shape future regulation.
In his view, future revisions to ADB should build on this research rather than introducing threshold changes that remain insufficiently supported by evidence and may prove difficult to undo.
The consultation closes on 1 July 2026, and Duckett argues that it warrants careful consideration from all those involved in the design and construction of timber buildings.
