From April 2020 HMRC is changing the rules related to the submissions of information and payment of Capital Gains Tax (CGT) due on the disposals of a UK residential property (other than a principle private residence). The tax due must be reported and paid to HMRC within 30 days of completion of the disposal. Non-UK residents who currently report property disposals within 30 days can no longer defer payment.

To enable customers to report and pay any CGT liability arising from gains on the sale of a property HMRC are developing a new digital service accessible from GOV.UK, which will be available from April 2020 to make it easier for customers to report and pay their CGT property disposal liability.
For many customers this will mean that if they have no other Self Assessment criteria, they will no longer need to register for Self Assessment to notify and pay for a ‘one-off’ property disposal to report the gain.
If your client is already within Self Assessment to report other liabilities you will need to ensure that the gain is also included on their Self Assessment tax return, HMRC will be amending the Self Assessment return to allow you to do this.
Late filing and late payment penalties will apply, and consequent interest will also continue to be charged.

source: Agent Update Issue 76