{"id":316,"date":"2026-07-27T09:00:00","date_gmt":"2026-07-27T09:00:00","guid":{"rendered":"https:\/\/taxbull.co.uk\/blog\/?p=316"},"modified":"2026-07-27T09:00:00","modified_gmt":"2026-07-27T09:00:00","slug":"self-assessment-investors-guide","status":"publish","type":"post","link":"https:\/\/taxbull.co.uk\/blog\/self-assessment-investors-guide\/","title":{"rendered":"Self-Assessment for Investors \u2014 A Complete Walkthrough"},"content":{"rendered":"

If you’ve sold shares at a profit, received dividends above \u00a3500, 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.<\/p>\n

Do you need to file?<\/h2>\n

You must register for self-assessment if you have untaxed income to report. For investors, the common triggers are:<\/p>\n

Capital gains above \u00a33,000 or disposal proceeds above \u00a312,000 (see our reporting thresholds guide<\/a>).
\nDividend income above \u00a310,000 \u2014 or above \u00a3500 if you have other reasons to file.
\nForeign income (including US dividends where you want to claim the foreign tax credit<\/a>).
\nYou want to claim capital losses to carry forward.<\/p>\n

HMRC’s online tool<\/a> can help you check whether you need to file.<\/p>\n

Registering for the first time<\/h2>\n

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

Registration takes up to 10 working days \u2014 HMRC sends you a Unique Taxpayer Reference (UTR) by post. Don’t leave it until January.<\/p>\n

Key deadlines<\/h2>\n\n\n\n\n\n\n\n
Deadline<\/th>\nWhat<\/th>\n<\/tr>\n<\/thead>\n
5 October 2026<\/td>\nRegister for self-assessment (if new)<\/td>\n<\/tr>\n
31 October 2026<\/td>\nPaper return deadline (rarely used now)<\/td>\n<\/tr>\n
31 January 2027<\/td>\nOnline return + payment deadline for 2025\/26<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Late filing attracts an automatic \u00a3100 penalty. After 3 months, daily penalties of \u00a310\/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<\/a> has the full schedule.<\/p>\n

Which forms do investors need?<\/h2>\n

SA100<\/strong> \u2014 The main tax return. Everyone files this. It covers employment income, pensions, and basic details.<\/p>\n

SA108<\/strong> \u2014 Capital Gains Summary. This is where your share and option gains go. Our box-by-box SA108 guide<\/a> walks through every field. Listed shares in Boxes 23-30, options<\/a> and crypto<\/a> in Boxes 14-22.<\/p>\n

SA106<\/strong> \u2014 Foreign Income. If you’re claiming foreign tax credit relief on US dividends (W-8BEN withholding), this is where it goes.<\/p>\n

HMRC’s online filing system adds these supplementary pages automatically when you tick the relevant boxes at the start of the return.<\/p>\n

The filing process<\/h2>\n

Step 1: Gather your data.<\/strong> Download CSVs from all your brokers. Run your CGT calculation \u2014 either manually using the matching rules<\/a> or via TaxBull<\/a>. Note your total gains, losses, proceeds, and costs.<\/p>\n

Step 2: Log in to HMRC.<\/strong> Go to gov.uk self-assessment login<\/a>. You need a Government Gateway account linked to your UTR.<\/p>\n

Step 3: Fill in the return.<\/strong> Start with SA100 (income, pension, employment). Then SA108 (capital gains \u2014 enter the box figures from your CGT calculation). Then SA106 if claiming foreign tax credits.<\/p>\n

Step 4: Review and submit.<\/strong> The system calculates your total tax liability. Check it makes sense. Submit online.<\/p>\n

Step 5: Pay.<\/strong> Payment is due by 31 January. You can pay by bank transfer, direct debit, or debit card. HMRC’s payment options page<\/a> lists all methods.<\/p>\n

Common mistakes investors make<\/h2>\n

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

Including ISA trades.<\/strong> Only GIA disposals count. Filter out ISA transactions before calculating. See ISA vs GIA<\/a>.<\/p>\n

Forgetting to claim losses.<\/strong> If you made losses, report them \u2014 even if you don’t owe tax this year. Carried-forward losses<\/a> can save you money in future years.<\/p>\n

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

Keep your records<\/a> for at least 5 years after filing. HMRC can open enquiries for up to 4 years (or longer if they suspect carelessness or fraud).<\/p>\n

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.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

A step-by-step guide to filing self-assessment as a UK investor. Covers registration, which forms you need, key deadlines, and how capital gains and dividends fit together.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[157,155,10,156,154,11,27,153],"class_list":["post-316","post","type-post","status-publish","format-standard","hentry","category-cgt-guides","tag-deadline","tag-filing","tag-hmrc","tag-investors","tag-sa100","tag-sa108","tag-self-assessment","tag-tax-return"],"_links":{"self":[{"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/posts\/316","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/comments?post=316"}],"version-history":[{"count":1,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/posts\/316\/revisions"}],"predecessor-version":[{"id":361,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/posts\/316\/revisions\/361"}],"wp:attachment":[{"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/media?parent=316"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/categories?post=316"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/tags?post=316"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}