This paper is aimed at analysing the levelised cost of energy (LCOE) of onshore wind turbine generators (WTGs) that are in operation beyond their design lifetime. In order to do so, the LCOE approach is introduced and input parameters are discussed for a UK deployment. In addition, a methodology is presented to support economic lifetime extension and investment decision making at the end of an asset's design lifetime. As part of a case study, a wind farm consisting of six 900 kW WTGs is subjected to different combinations of i) lifetime extension (5-15 years), ii) input assumptions (pessimistic, central, optimistic), and iii) reinvestment types (retrofits). Results indicate that in the central lifetime extension scenario, LCOE estimates of 22.40 /MWh are achievable. (C) 2017 The Author(s). Published by Elsevier Ltd.