Q: Do I need to use the cavity widening kit with the H30 Hideaway pocket door?

A: (from Doorstuff) You only need to use it if the door is between 36 and 44mm thick.  Without the cavity widening kit, your finished wall thickness will be 115mm, however you can use the cavity widening kit to increase your finished wall thickness to 120mm whilst using a 35mm thick door (all based on 2 x 1/2″ thick plasterboards).  It is included in the kit.