Generally, a visitor visa cannot be extended. However, in exceptional circumstances, you may do.
Note 2 key points:
1) apply before the existing visa expires so you are not caught by the ‘breach’ provisions;
2) if you are refused (it takes on average 6 months to get a decision anyway so you may end up with the extension by default), do consider leaving immediately so that you do not jeopardise future applications.
See Part V8 of the Immigration Rules which says:
PART V8. EXTENSION OF STAY AS A VISITOR
Who can apply for an extension of stay as a visitor
V 8.1 It is not possible to switch to become a visitor while in the UK where a person is in the UK in breach of immigration laws or has entry clearance or leave to enter or remain for another purpose.
Making an application for an extension of stay as a visitor in the UK
V 8.2 An application for an extension of stay as a visitor must comply with the requirements in paragraphs 34 – 34C of Part 1 of these Rules.
Eligibility requirements for an extension of stay in the UK as a visitor
V 8.3 The applicant must be in the UK as a visitor. Visitors for permitted paid engagements and transit visitors may not apply for an extension of stay as a visitor.
V 8.4 An application for an extension of stay as a visitor must satisfy the decision maker that they continue to meet all the suitability and eligibility requirements for a visit visa.
V 8.5 The applicant must not be in the UK in breach of immigration laws, except that where paragraph 39E of these Rules applies, any current period of overstaying will be disregarded.
V 8.6 If the applicant is applying for an extension of stay as a visitor for the purpose of receiving private medical treatment they must also satisfy the decision maker they:
(a) have met the costs of any medical treatment received so far; and
(b) provide a letter from a registered medical practitioner, at a private practice or NHS hospital, who holds an NHS consultant post or who appears in the Specialist Register of the General Medical Council, detailing the medical condition requiring further treatment.
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