Surinder Singh applications are for family members of British citizens (other than dual EEA/British nationals) who spent time living in another EEA country or Switzerland with their British family member, who could be:

A spouse or civil partner;
A child or grandchild under 21 years old;
A dependent child or grandchild aged 21 years or older; or
A dependent parent or grandparent.

Requirements to apply under the Surinder Singh route

The British family member must have genuinely made their home in the other EEA country. Both have to show that:

that country was your main residence or base for the ‘centre of your life’
you lived there together
you had integrated there

The British family member must either have the right to permanent residence in the EEA country where you’ve lived together, or have exercised treaty rights as a worker, self-employed person, student or self-sufficient person in accordance with the EEA rules, in that country.

If the British family member has been back in the UK for more than 3 months, they must also be working, looking for work, self-employed, self-sufficient or studying in the UK.

VERY IMPORTANT: The deadline to return to the UK with the family member and be eligible to apply via this route is 29 March 2022.

Evidence to be provided

Evidence of family relationship, such as a marriage/civil partnership certificate or birth certificate
Details of time spent in the UK (arrival and departure dates)
Details of any visa or immigration applications made
Address(es) in the other EEA country and evidence of living together in that country, such as proof of buying or renting a home.
Evidence of integration into the local community, proof of speaking the language, having children born or living there
Proof that the British family member was working, self-employed, self-sufficient or studying in the EEA country where you’ve lived together
Proof that the British family member is working, looking for work, self-employed, self-sufficient or studying in the UK, if they returned more than 3 months ago.

Examples of proof include employer’s letters, wage slips, contracts, bank statements, proof of tax registration, or proof of enrolment and attendance for study.

A certified translation of all documents that are not in English is required.

You cannot apply online under this route

Request a paper form: https://eu-settled-status-enquiries.service.gov.uk/start
Dual UK/EEA nationals can apply under Surinder Singh. This is confirmed
here.

Sources:

https://www.gov.uk/settled-status-…/not-EU-EEA-Swiss-citizen
https://www.gov.uk/family-permit/surinder-singh
https://www.gov.uk/apply-for-a-uk-residence-card/eligibility
https://assets.publishing.service.gov.uk/…/family-member-of…