Approved Document B requires compartmentation in virtually every multi-storey or multi-occupancy building in England and Wales.
For dwellings, Volume 1 requires compartment walls and floors between every dwelling unit and between dwellings and common areas. In blocks of flats over 11 metres, external wall cavities must carry limited combustibility cavity barriers. Over 18 metres, A2-s1,d0 or better classification is required for all external wall products. These height thresholds were reinforced following the post-Grenfell review of the 2022 edition.
For non-residential buildings, Volume 2 sets maximum compartment sizes by use class, with requirements varying according to the purpose group of the building. Hospitals, care homes and educational buildings carry additional requirements under HTM 05-02 and BS 9999, which sit alongside and extend the baseline ADB provisions.
Higher-risk buildings under the Building Safety Act 2022, defined as residential buildings of 18 metres or 7 storeys, trigger the full BSR gateway regime, requiring a documented fire strategy at each stage. Beyond legal minimums, compartmentation is also required by insurer mandates on commercial and industrial buildings, and by developer standards on mixed-use schemes.