Section J 2025 — what actually changed for MEP consultants

The 2025 NCC update tightened Section J DTS thermal performance targets by roughly 15% and introduced a reframed Performance Solution pathway that gives designers more flexibility on how to demonstrate compliance — provided the modelling and documentation stand up to third-party review.

For most commercial and multi-residential projects, the DTS path is now measurably harder to hit without upgrading glazing specifications or increasing insulation depths. On heritage or retrofit projects, the Performance Solution path becomes the practical option — but the documentation burden has grown.

The change that most impacts our workflow is the requirement for explicit assumptions around occupant density and internal gains. Where the previous version allowed default values, the 2025 version expects assumptions to be defensible against the specific building use case. That's a documentation change more than an engineering change — but it adds cost if it's discovered late.

Practically: bring your Section J consultant in during schematic design, not documentation. The economic penalty for late intervention is larger under the 2025 code than it was previously.