Capital Gains Tax (CGT)
A Capital Gains Tax (CGT) return should be filed whenever you sell, or dispose of a Capital Asset, such as a house, land or business assets such as shares in a trading company. The amount of tax may be very little or even nil, but the return should still be filed.
A Capital Asset is something that you purchase to keep, and is expected to be worth more when you sell it than when you purchased it. Sometimes the asset has lost value, and in this case you still file the return when you dispose of the asset, but there is no tax to pay and the loss can be put against other gains on other disposals.
The Capital Gains Tax (CGT) return should be filed within 30 days of the completed sale or disposal, in the case of property disposal the HMRC requires the return filed online within 30 days any tax paid, otherwise interest may be charged.
If you are selling a property and have engaged a solicitor, they will often calculate your CGT for you, some will file it on your behalf, others will expect you to hire an accountant or advisor to do this for you.
You can work out your Capital Gain and the tax due here:- https://www.gov.uk/tax-sell-property/work-out-your-gain