Some people are entitled to British citizenship from the moment of birth; others must acquire it. It can be acquired by naturalisation (for adults aged 18 or over) or registration (typically for children but in some cases for adults).
There are two different routes for naturalisation: a five-year route and a three-year route. Under the five-year route a person must have been legally in the UK for at least five years and they must have held indefinite leave to remain/permanent residence for at least one year within the period.
Under the three-year route a person must have been legally in the UK for at least three years, be married to a British citizen, and they must hold indefinite to remain/permanent residence when they apply (but they do not have to have held if for any particular period of time).
There are various other requirements, the chief ones of which are that the applicant must not have spent too much time outside the UK during the qualifying period, they must not have any unspent criminal convictions, and they must have satisfied an English language requirement and passed the Life in the UK test.
There is also a requirement for successful applicants to attend a Citizenship Ceremony, which is organised by local authorities. Applicants must attend this ceremony for their British citizen status to be confirmed.
The requirements for registration of children as British citizens are more flexible, do not depend on rigid time periods, and there is no English language requirement or Citizenship Ceremony. Most relevant to the application is the immigration status of the parents, and the intentions of the parents for the future. Parents and children may make a joint family application for British citizenship.
Nationality Checking Service
British citizenship applications may be made to the Home Office by post, for which it is necessary to include passports and in many cases other original documents such as biometric residence permits. The Home Office will retain these documents until they have made a decision on the application, which is likely to take several months.
For applicants who do not wish to be deprived of their original documents there is a service available called the Nationality Checking Service, which is run by local authorities. The applicant can make an appointment, bring their completed application form and documents with them, and have their application initially assessed. A photocopy of all their original documents is taken and the original documents returned to them, and then the application is forwarded to the Home Office for them to process it in the normal way. There is an extra fee for this service.
So the applicant can keep their original documents so that, for example, they can travel outside the UK while they are waiting for their application to be processed. This service also confers the benefit that the application is initially assessed, and if there are any problems with it they are identified. This will prevent a weak application from being submitted and likely being refused by the Home Office, so this can be a very good option for some applicants.