In case, your visa for temporary stay in UK is about to expire or you want to extend your stay in the country. You can apply for extending the period of the visa. Well, there are provisions for extension as per the visa that you have applied to. If you are on a visitor visa, then as per the regulation, you cannot extend your stay in the country. Though in few instances, you need to switch to a different category of visa.
If you are planning to extend your stay, then you must apply for the same prior to the expiry of the existing one. In case the duration of your visa has expired, then you need to leave the country or you will be removed by the authorities. The country has also imposed law on the extension of the student visa. While applying for a visa, you need to ensure that you satisfy various requirements of your immigration category.