From April 2020, the way that you are required to report and pay capital gains tax made from the sale of residential property is changing.
At present, gains made by individuals are reported through self-assessment. i.e. if you sell a property in the tax year ended 5 April 2020, you must declare it on your tax return and pay the tax you owe no later than 31 January 2021.
The new rules:
As of 6 April 2020, you will have just 30 days following completion of the sale to submit a provisional calculation of the gain and pay the tax you estimate is due.
You must still declare the gain on your self-assessment tax return and pay, by the usual self-assessment deadline, any gain over and above what you estimated.
Once you have submitted your estimated calculation and paid the tax, if you discover you made a capital loss, you will not be allowed to reduce it until you submit your self-assessment tax return. There will be penalties if you do not meet the 30 day deadline for reporting and paying the tax.