Which NRCGT calculation method should I use?

Method A — Rebasing to 5 April 2015. Only gains accruing after that date are taxable. This is the default for residential property held by non-residents before the 2015 NRCGT extension. Usually the right answer where property values grew relatively steadily throughout the ownership period.

Method B — Straight-line time apportionment. The whole-period gain (from purchase to sale) is multiplied by the fraction of ownership falling after 5 April 2015. This method wins when post-2015 value growth was disproportionately strong — the time-apportionment calculation effectively dilutes that rapid growth across the full ownership period, producing a smaller taxable figure than rebasing.

Method C — Whole-period gain at original cost. The full gain since purchase is taxable with no rebasing relief at all. Almost never the right choice unless the property has actually depreciated since 2015, in which case Method C may give a smaller gain than rebasing to a higher 2015 value.

A credible 5 April 2015 valuation is essential for Methods A and B. If you didn't obtain a valuation at the time, a RICS-qualified retrospective valuation (based on Land Registry comparables and contemporaneous estate-agent records) typically costs £400–£900 and is the standard evidence HMRC accepts. Unsupported valuations are a common HMRC enquiry trigger.

60-day NRCGT return is unconditional. Non-residents must file an NRCGT return within 60 days of completion regardless of whether any tax is due — and regardless of whether the disposal is a loss. This is different from the rule for UK residents (who only file when tax is payable). Missing the 60-day deadline triggers automatic £100 penalty, escalating to £300+ at day 91 and further daily penalties from month 7.

Treaty relief coordination. Most UK double taxation treaties give the country where the property sits (the UK) the primary taxing right, but allow foreign tax credits against your resident-country tax. Pay UK first, claim the credit abroad. Getting the sequencing wrong can produce double taxation that is difficult or impossible to unwind. A specialist coordinates the UK filing with the overseas filing.