Severe Disablement Allowance is not payable for claims made after April 2001. It was designed to help people who had never been able to work, or who had not been able to work for at least 28 weeks, because of physical or mental illness or disability.
You cannot make a new claim for SDA. You can only get Severe Disablement Allowance now if you were already getting it on 5 April 2001 or claimed later when linking rules applied. You may not get Severe Disablement Allowance paid to you if you have another benefit paid at a higher amount. This is due to the rules on overlapping benefits.