There is nothing in the Council Tax Discount SMI Disregard pocedure that requires a formal hospital diagnosis of dementia, it only requires a GP to certify when
in their opinion they meet the requirements of SMI as in the LGFA 1992.
The LGFA 1992 Schedule 1 section 2(2) states :-For the purposes of this paragraph a person is severely mentally impaired if he has a severe impairment of intelligence and social functioning (however caused) which appears to be permanent.
The laws as they stand only require a GP (or other medical practitioner) to certify the date.
To confuse the issue even further councils often refer to this as the "date of diagnosis".
Councils vary in their interpretation of the laws. Of the 382 UK councils questioned through a FOI request, 127 said that they backdate to the date of GP diagnosis/certification and 255 said they wait until the date they are also in receipt of a qualifying benefit. Thus the petition
https://petition.parliament.uk/petitions/614616