Self-Assessment for Investors — A Complete Walkthrough

If you’ve sold shares at a profit, received dividends above £500, or need to report capital losses, you’ll probably need to file a self-assessment tax return. For many investors, this is the first time they’ve had to deal with HMRC directly. It’s less daunting than it looks.

Do you need to file?

You must register for self-assessment if you have untaxed income to report. For investors, the common triggers are:

Capital gains above £3,000 or disposal proceeds above £12,000 (see our reporting thresholds guide).
Dividend income above £10,000 — or above £500 if you have other reasons to file.
Foreign income (including US dividends where you want to claim the foreign tax credit).
You want to claim capital losses to carry forward.

HMRC’s online tool can help you check whether you need to file.

Registering for the first time

If you’ve never filed self-assessment before, you need to register with HMRC before 5 October following the end of the tax year. For 2025/26 (ending 5 April 2026), register by 5 October 2026.

Registration takes up to 10 working days — HMRC sends you a Unique Taxpayer Reference (UTR) by post. Don’t leave it until January.

Key deadlines

Late filing attracts an automatic £100 penalty. After 3 months, daily penalties of £10/day kick in (up to 90 days). After 6 months, an additional 5% of tax owed. After 12 months, further penalties. The HMRC penalties page has the full schedule.

Which forms do investors need?

SA100 — The main tax return. Everyone files this. It covers employment income, pensions, and basic details.

SA108 — Capital Gains Summary. This is where your share and option gains go. Our box-by-box SA108 guide walks through every field. Listed shares in Boxes 23-30, options and crypto in Boxes 14-22.

SA106 — Foreign Income. If you’re claiming foreign tax credit relief on US dividends (W-8BEN withholding), this is where it goes.

HMRC’s online filing system adds these supplementary pages automatically when you tick the relevant boxes at the start of the return.

The filing process

Step 1: Gather your data. Download CSVs from all your brokers. Run your CGT calculation — either manually using the matching rules or via TaxBull. Note your total gains, losses, proceeds, and costs.

Step 2: Log in to HMRC. Go to gov.uk self-assessment login. You need a Government Gateway account linked to your UTR.

Step 3: Fill in the return. Start with SA100 (income, pension, employment). Then SA108 (capital gains — enter the box figures from your CGT calculation). Then SA106 if claiming foreign tax credits.

Step 4: Review and submit. The system calculates your total tax liability. Check it makes sense. Submit online.

Step 5: Pay. Payment is due by 31 January. You can pay by bank transfer, direct debit, or debit card. HMRC’s payment options page lists all methods.

Common mistakes investors make

Using US broker tax reports. If you use Robinhood or Tastytrade, their 1099 forms use US rules (FIFO, wash sales). Ignore them entirely for your UK return. See our UK vs US rules comparison.

Including ISA trades. Only GIA disposals count. Filter out ISA transactions before calculating. See ISA vs GIA.

Forgetting to claim losses. If you made losses, report them — even if you don’t owe tax this year. Carried-forward losses can save you money in future years.

Missing the registration deadline. If 2025/26 is your first year with capital gains, register by 5 October 2026. Don’t wait until January.

Keep your records for at least 5 years after filing. HMRC can open enquiries for up to 4 years (or longer if they suspect carelessness or fraud).

This is general information about the self-assessment process. For complex returns, consider using an accountant or tax software. HMRC’s helpline (0300 200 3310) can assist with technical filing questions.