There are two allowances that will be changing from April 2020 onwards. These only affect people who have taxable gains on their main residence.
1) Lettings relief – this is an additional relief available (up to a limit of £40,000) to those who have lived in a property as their main residence that is being sold and has also been let. From April 2020 the relief will only be available to those who are in shared occupancy with a tenant.
2) Final period exemption – at present, the final 18 months of ownership is always taken to be exempt irrespective of the actual situation. From April 2020, this is reduced to 9 months in the calculations. (The original intention of the relief was to exempt gains during the period while people sold a property after already moving into a new one.)