Proposed UK Building Regulation Changes Could Restrict Mass Timber Construction

On Tuesday, March 25, the UK government published proposed changes to Approved Document B (ADB), the statutory fire safety guidance for new buildings in England. The consultation remains open through July 1.

The proposal would require all loadbearing structural elements in buildings with an occupiable floor more than 11 meters above ground to achieve a minimum reaction-to-fire classification of A2-s3, d2 or better. Timber cannot meet that classification, meaning the change would effectively prohibit the use of structural timber in many low- and mid-rise buildings.

Critics argue the proposal would expand the definition of higher-risk buildings based solely on structural material rather than occupancy type or fire risk. If adopted, they say the changes would largely eliminate structural timber from many schools, urban housing developments, rooftop extensions, laboratories, and office buildings that currently represent key markets for mass timber construction in the UK.

For buildings between 4.5 and 11 meters tall, the proposal would also impose additional restrictions on exposed combustible surfaces. Opponents contend the requirements would reduce many of the architectural and biophilic qualities associated with timber construction while requiring additional fire-resistant finishes and related detailing.

The UK government cited BRE Global research published in September 2025 as the basis for lowering the threshold from 18 meters to 11 meters. Critics argue the report does not clearly justify the lower threshold and relies heavily on observations from a single cross-laminated timber (CLT) compartment fire test that was not designed to evaluate smoldering behavior. They also contend the proposal fails to distinguish between occupancy types and risk categories, noting that modern mid-rise buildings would typically be evacuated within minutes of a fire alarm.