Self-certification enables you to ask for additional consideration without the need to provide medical evidence, when you have suffered short-term ill-health that does not require medical intervention. This is in-line with arrangements in most workplaces, where employees are generally allowed to report their own absences and to self-certify short periods of illness.
You may self-certify extenuating circumstances that affect you for seven days or fewer. Self-certifying means that you must state what your extenuating circumstances are, but you do not need to provide further evidence of the circumstances themselves. As well as describing the circumstances, we will ask you to tell us, in your claim, how they have impacted your ability to attend and complete assessments. When self-certifying, the claim must be received immediately following the seventh day.
Valid extenuating circumstances must meet all three of the following criteria:
Claims that do not meet this definition will be considered invalid.
If considered valid, your self-certified extenuating circumstances claim is likely to result in you having:
Even with an extension granted, you will still need to submit your assessment by the newly agreed timeframes.
If you wish to defer your assessment, then you will need to submit a separate deferral claim, which will require supporting evidence to be provided
See more here for Extenuating Circumstances