NHBC technical position on disproportionate collapse and transitional arrangements

For new sites registered from 1 July 2025, NHBC will no longer recognise use of the Building Control Alliance (BCA) Technical Guidance Notes 21¹ and 22² as mean of justification for a disproportionate collapse design.

Where designs addressing disproportionate collapse do not follow the principles outlined in Approved Document A3, NHBC will require robust justification supporting the design methodology to be submitted, demonstrating how the Building Regulations requirement is being achieved.

Any sites registered prior to 1 July 2025, which have plots or blocks based on BCA Technical Guidance Notes 21 and 22 will need to have foundations completed by 1 January 2026. Where plots or blocks do not achieve this, it will be necessary to re-justify the disproportionate collapse approach following the new technical position detailed above.

For foundations to be judged as completed it is required that:

• all excavations for trench fill or strip footings have been completed and fully concreted,

• piling works should be completed with concreted ground beams,

• raft foundations should be fully concreted.

All previous guidance published by NHBC in respect of disproportionate collapse should be considered superseded with immediate effect.

¹BCA Technical Guidance Note 21 – The Building Regulations 2010 – England and Wales Requirement A3 – Disproportionate Collapse.

²BCA Technical Guidance Note 22 – Assessment of proposed robustness measures following the ‘alternative approach’ given in Approved Document A Section 5 paragraph 5.4.