If you have not returned the questionnaire within three weeks, you should be sent a reminder. If you do not return the questionnaire within a week of receiving the reminder, you will be treated as not having a limited capability for work unless you can show you had ‘good cause’ for failing to return it (or in the case of universal credit, an undefined ‘good reason’).
ESA Regs , regs 18 & 34; UC Regs, reg 43(3)-(4)

Good cause – When deciding whether you had good cause for failing to complete and send back the questionnaire, the DWP must take into account your health, your disability and whether you were outside Great Britain. However, other reasons could be valid. You have a right of appeal against a decision that you failed to send back a completed questionnaire, but you cannot be paid the basic allowance of ESA pending an appeal on this matter (see 18 below ).
ESA Regs , regs 20 & 36

What happens next? – Once the DWP have received your completed questio