Once a person has entered the UK they can remain for the duration of their leave as stamped in their passport or set out in their entry clearance. Prior to the expiry of their leave they can apply to the Immigration and Nationality Directorate for an extension of stay in accordance with the requirements of the Immigration Rules. So long as the application is made before current leave expires the applicant will be lawfully in the UK until a decision is made. The Immigration Rules set out the circumstances in which extensions of stay may be granted. It is possible in certain circumstances to switch categories but certain nationalities are not allowed to switch. If the Rules do not allow such a switch, the individual concerned has to leave and apply for an entry clearance from abroad to return.

Applications for an extension of stay have to be made on the correct form and be supported by original documents. There are special forms for a variety of categories, including students, married and unmarried partners, family members, and EEA nationals. 

Some categories of the Immigration Rules may eventually lead to permanent residence (e.g. marriage and some work permits) but others have an expectation that the applicant will eventually return home (e.g. visitors, short-term students). Ten years of lawful continuous residence can lead to permanent stay and 14 years of continuous residence may lead to permanent stay even if some of the time has been unlawful.