The chargeable gain is broadly the sale price less the acquisition cost, buying/selling costs (agent, legal, SDLT) and qualifying capital improvements. Private Residence Relief removes CGT on the home you actually live in; a second home, buy-to-let or overseas property sold by a UK resident is within scope.
A UK residential property disposal must be reported and the tax paid to HMRC within 60 days of completion via a CGT-on-property-account return — a deadline buyers and sellers routinely miss. Non-residents disposing of UK property are also within scope and face the same 60-day reporting, regardless of whether a gain arises.
For internationally mobile owners, the interaction of CGT with the buyer's country of residence and any double-tax treaty matters: the same gain can be taxable in two places with relief given for the tax paid in one. Model the figure before you exchange, not after.