Any non-EEA national who has remained in the UK beyond the validity of their visa (without having submitted a valid application for extension prior to expiry) is considered to be an 'overstayer'.
In a tightening of the Immigration Rules announced on 16 March and due to come in to force on 6 April 2017, the government has now reduced the period of time of 'accepted' overstaying that would not lead to a significant re-entry ban.
Paragraphs 320(7B)(i) and Appendix V paragraph 3.9(a) are being amended to the effect that unless specific exceptions apply, anyone who has overstayed for more than 30 days will be subject to a 12 month re-entry ban. Prior to the change, this period of time was 90 days, which could be considered to be a far more reasonable period in which a person might be able to wind up their affairs in the UK before a departure. The government cite a need to increase compliance with the Immigration Rules and reduce overstaying as the driving forces behind this change.
There is some protection for those who have overstayed for over 30 days and are leaving the UK to apply for entry clearance as a family member. In particular, paragraph A320 offers some protection to those making an application under Appendix FM from refusal on the grounds of overstaying.
Any person overstaying for more than 30 days, as of 6 April 2017, will now face a re-entry ban of 1 year, provided they departed from the UK voluntarily at their own expense. For further information on re-entry bans, Free Movement have published a helpful infographic describing the various levels of ban from 0 up to 10 years: https://www.freemovement.org.uk/general-grounds-refusal-understanding-re-entry-bans/