There are two deflection situations stated in Approved Document B, walls mid-span of a floor, and walls under beams.

For Walls mid-span of a floor, 40mm deflection may be assumed, but can be reduced through assessment and reduced linearly when positioned closer to the floor’s supporting structure. Structural fire engineers can assess the likelihood of significant floor deflection during fire conditions exceeding their cold state calculations.

For walls under beams, the non-fire deflection tolerances can be used as testing by SCI and BRE, specifically referenced in Approved Document B, has shown (in document P288, extract below) that the beams do not significantly deflect when heated from one side only (i.e. in a fire compartment situation).

Approved Document B:

SCI P288: