Self assessment, the SA108 helper box by box

This page is a stub. The full guide is written against the shipped app version and is on its way.

Cryptoasset disposals belong in the Other property, assets and gains section of the SA108, the Capital Gains Tax summary pages of the self assessment return. The helper report outputs one sheet per tax year with the figures ready to copy.

The boxes, verified against the actual SA108 2025 form. Box 23, number of disposals. Box 24, disposal proceeds. Box 25, allowable costs including the purchase price. Box 26, gains in the year before losses. Box 27, losses in the year.

Rounding never works against you. Whole pounds, with proceeds and gains rounded down and costs and losses rounded up, the direction HMRC accepts and the helper applies consistently.

The two loss figures come labelled, not numbered. Losses brought forward and used this year, and losses available to carry forward, belong in the Losses and adjustments section, whose box numbering varies by year. The helper gives them as labels on purpose, match the label to the form in front of you. Filing for a year other than 2025, check that year’s numbering before copying, the helper says the same on its face.

The caveat that belongs here verbatim. Only file from figures whose reconciliation you trust. The working paper carries the audit trail behind every number, each disposal with its matching rule cited, and the helper’s output is a computation, not a filed return and not personal tax advice.

The rest of the filing kit. The capital gains report lists every disposal with proceeds, cost, gain and rule. The negligible value claim letter, when your data holds a case for one, is a section 24(2) TCGA 1992 template with the cost figures highlighted for you to confirm. Keep the session file alongside the return, record keeping covers the habit.