When you sell a residential property that is not your main home, the capital gains tax is not settled through your next Self Assessment. It must be reported, and paid, within 60 days of completion, on a standalone HMRC return most sellers have never heard of until it is late.
The calculation has more moving parts than people expect: acquisition costs, capital improvements (not repairs; the distinction matters), selling costs, lettings history, and any periods the property was your main residence, which can qualify slices of the gain for relief.
Timing is a genuine lever. Straddling a tax year, using both spouses’ annual exemptions, offsetting brought-forward losses, or sequencing two disposals across different years can each change the bill materially, but only if the planning happens before exchange.
The 60-day return also interacts with your ordinary Self Assessment: payments made in-year are credited against the final liability, and getting the estimates right avoids both penalties and unnecessary overpayment.
Our rule for landlord clients is simple: call us when you are thinking about selling, not when you have sold. An hour of planning before the listing is worth more than any amount of accounting after completion.