An ESPP purchase buys shares at a discount to market value; the discount is taxed as pay, through payroll, on the purchase date. What counts as the shares' cost for UK Capital Gains Tax is not simply what you paid.

The rule

For an ordinary, non tax-advantaged ESPP, the allowable cost is the market value of the shares on the date you acquired them (Taxation of Chargeable Gains Act 1992 s17; HMRC helpsheet HS287, Capital Gains Tax and employee share schemes, §11) — market value, not the discounted price actually paid.

Why price paid plus the discount taxed usually gives the same answer

Where the discount taxed through payroll really is market value at purchase less the price paid, adding the two figures back together reconstructs market value exactly. That is the ordinary case, and it is why "price paid" plus "discount taxed" so often agrees with market value at purchase.

Where they diverge

Some plans tax a discount calculated from an earlier, lower "look-back" price rather than the purchase-date price. There, the amount taxed through payroll understates market value at purchase, and price-paid-plus-discount no longer equals it. The allowable cost for Capital Gains Tax is still market value on the purchase date; the payroll figure is evidence of it, to be checked against market value rather than adopted on its own.

One thing this does not cover

A restricted or convertible ESPP share — one with a lock-up period — is not measured this way; the cost is the actual amount paid, under a different rule (Taxation of Chargeable Gains Act 1992 s149AA). A locked-up ESPP is not something this app computes.

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