Further Leave to Remain

To remain in the UK for an extended amount of time, you must apply for an extension of stay or a new visa before the expiration of your current visa to avoid overstaying, unless certain specific exceptional criteria apply. This extension is known as ‘further leave to remain’ (FLR).

In the case of Visitor Visas, you cannot extend your stay past the maximum of 6 months, and should apply for a more appropriate visa if you wish to stay. Again, this is unless there are exceptions reasons. If staying in the UK based on work, your employer will be required to register as a sponsor to acquire FLR for you as an employee.
If your visa was gained through the points based system, you can apply for FLR in the case that your studies or employment will continue as a result of the approval.

Alternatively, you may be eligible for Leave on a discretionary basis, which is often refered to as Discretionary Leave to Remain. This can be obtained in two ways, being if you have claimed asylum in the UK and were not recognised as a refugee by the Home Office or through the submission of a specific application. The Home Office may grant you this temporary permission to stay in the UK, with the period of your stay dependent on your situation, but usually for 30 months on the 10 year route to settlement.
You can apply for DLR as a result of specific Rules which exist, which are based on Article 8 of the European Convention on Human Rights (ECHR), which argues that a person’s removal from the country will breach of the UK’s commitment to holding refugees under Article 8 of the ECHR. The DLR is granted by the Secretary of State, an unbiased judgement outside the Immigration Rules, who will take all relevant information, both factual and emotional, into consideration. The Secretary of State will make the final decision on whether to grant you Discretionary Leave to Remain.

To apply for both FLR and DLR, the FLR(HRO), FLR (FP) and FLR(IR) application forms, for which formerly the single FLR(O) form existed, must be used. The FLR(HRO) form is a document used for applications relating to human rights breeches and claims, leave that acknowledges a change in the usual rules, and situations not covered by other application forms. The FLR (FP) form is for an application based on family relationships, such as through marriage or children, and where removal would cause undue hardship or problems for the family unit.

The FLR(IR) form is used for applications for other routes under the Immigration Rules. This can be used for visitors to the UK, domestic workers who are victims of slavery or working in a household, people with UK ancestry, members of the Armed Forces, parents of children living in the country, business representatives, and retired persons. By applying for either leave, you can avoid overstaying.

Overstaying is the term given to staying in the country for any period with an expired visa, whether your visa expired while in the country or outside. Overstaying is a criminal offence, and there is no lenience or lawful reason for overstaying.
Though there is no period after the expiration for you to lawfully remain in the country and apply for FLR, if you submit a suitable application before your valid visa expires, but it expires while the outcome is still being decided, you will not be classed as an overstayer in most circumstances. Rather, your existing immigration status will be extended until the decision is made.
If your valid application is denied, you may have the right to appeal before an immigration judge, or alternatively you can attempt a judicial review.