The period from 1 April 1988 to 30 September 1993 is 66 months of which 36 months is classed as deemed occupation (3 years for any reason), therefore leaving 30 months as non occupation during which the house was let (information needed to subsequently apply the lettings relief). If you read note 4.4 on lettings relief, immediately prior to this example, you will see where the £40,000 comes from! This is not however the figure deducted, as the lettings relief is the lower of the 3 figures shown!