To fill out HMRC Form SA108, the Capital Gains Summary, you attach it to your SA100 Self Assessment return and enter each disposal in the section that matches the asset type, showing proceeds, allowable costs, and the resulting gain or loss, then apply any reliefs and losses before the figures feed into your main tax calculation. The form covers property, shares, cryptocurrency, and valuable possessions, and you either complete it inside HMRC’s online service or on paper alongside the SA100.
Do You Actually Need to File SA108
Not every disposal puts you on the form. There are three separate triggers, and any one of them is enough.
The first is the gains threshold. If your total taxable gains, after deducting losses, exceed the Annual Exempt Amount of £3,000 for 2025–26 onward, you must complete the SA108.1GOV.UK. Capital Gains Tax: What You Pay It On, Rates and Allowances – Capital Gains Tax Allowances
The second is the proceeds threshold. Even if your gains are small or nil, you must file if your total disposal proceeds across all sales exceed four times the Annual Exempt Amount — £12,000. This catches people who sold expensive assets at little or no profit.
The third is losses. If you want a capital loss to count in future years, you have to report it. Unreported losses vanish, and you have only four years from the end of the tax year in which the loss arose to notify HMRC. Miss that window and the loss is permanently forfeited.
Trustees use a smaller allowance of £1,500, rising to £3,000 where the beneficiary is a vulnerable person.2GOV.UK. Trusts and Capital Gains Tax
A disposal is not only a sale. Gifting, exchanging, or otherwise transferring a chargeable asset also counts, and HMRC treats a gift as taking place at market value. Transfers between spouses or civil partners are the main exception and happen at a no-gain, no-loss value.
What Goes in Each Section
The SA108 groups disposals by asset type. Each section follows the same pattern: number of disposals, total proceeds, allowable costs, and the resulting gain or loss.3GOV.UK. Capital Gains Tax Summary Notes 2025
- Boxes 3 to 13B: residential property and carried interest gains. Buy-to-let sales, second homes, inherited houses you never lived in, and holiday homes belong here.
- Boxes 14 to 22: other property, assets, and gains. Commercial buildings, agricultural land, development sites, and anything that does not fit the other categories.
- Boxes 23 to 30: listed shares and securities traded on recognised exchanges.
- Boxes 31 to 44: unlisted shares in private limited companies.
Cryptocurrency disposals are reported like any other chargeable asset, as are valuable personal possessions where the proceeds on a single item exceed £6,000. If proceeds are £6,000 or less on an item, the gain is exempt. Between £6,001 and £15,000, a marginal relief formula caps the taxable gain.4HM Revenue & Customs. Personal Possessions and Capital Gains Tax 2024 (HS293)
Wasting assets, meaning items with a predictable useful life under 50 years such as most machinery and vehicles, are generally exempt. The exception is assets that have qualified for capital allowances in a business, which stay chargeable.
For each disposal you also need a computation: the arithmetic showing proceeds minus acquisition cost minus allowable expenses. Online filers get walked through this by the software, which stores the working. Paper filers should attach a separate computation sheet for every disposal so HMRC does not have to come back with questions.
Working Out Each Gain
The taxable gain is proceeds minus allowable costs. Costs fall into three groups, and the more legitimate cost you can evidence, the smaller the gain.
Acquisition costs are what you originally paid, plus incidental buying costs such as solicitor and surveyor fees and Stamp Duty Land Tax on property. Enhancement expenditure is money spent improving the asset in a way still reflected in its value at disposal — a kitchen renovation on a rental property counts, routine maintenance does not. Disposal costs include estate agent fees, solicitor fees for the sale, advertising, and valuation fees for the sale itself.
Keep every receipt. If HMRC queries a claim and you cannot produce documentation, they disallow the cost and recalculate the tax upwards.
Valuations That Need Special Care
Two situations require market value rather than an actual price. First, gifts: the disposal proceeds are the market value on the date of the gift, not the nothing you actually received.5GOV.UK. Capital Gains Tax: Market Value Second, any asset you owned before April 1982 uses its market value as at 31 March 1982. HMRC can check valuations after the fact using a post-transaction valuation check (form CG34) and allows around three months to respond.
Negligible Value Claims
If an asset you still own has become worthless or nearly worthless, you can treat it as disposed of without selling it. Report the details in box 54 of the SA108 or in an attached computation.6HM Revenue & Customs. HS286 Negligible Value Claims and Income Tax Losses on Disposals of Shares (2025) The claim creates a deemed disposal and immediate reacquisition at the value you specify, generating a loss you can set against other gains. You can backdate the deemed disposal up to two years before the start of the tax year in which you make the claim, provided the asset was already of negligible value and you owned it at that earlier date.
Applying Reliefs and Losses
Reliefs and losses are what turn the raw gain into the taxable figure that ends up on your SA100.
Private Residence Relief
If a home was your only or main residence for the whole period you owned it, the full gain is exempt and you do not need to report the sale at all. The relief applies automatically when you lived there as your main home throughout, did not let any part of it (a lodger does not count as letting), did not use any part exclusively for business, the grounds are under 5,000 square metres, and you did not buy purely to make a profit.7GOV.UK. Tax When You Sell Your Home: Private Residence Relief Miss any of those conditions and a proportion of the gain becomes taxable. Married couples and civil partners can only nominate one main home between them at a time.
Letting Relief
If you qualified for at least some Private Residence Relief and you lived in the property at the same time as your tenants, Letting Relief may reduce the taxable portion attributable to the letting. It is the lowest of the Private Residence Relief you received, £40,000, or the chargeable gain from the letting period.8GOV.UK. Tax When You Sell Your Home: If You Let Out Your Home Shared occupancy is essential; letting a property after moving out does not qualify.
Business Asset Disposal Relief
Formerly Entrepreneurs’ Relief, this charges a reduced 14% rate on qualifying business disposals up to a £1 million lifetime limit.9HM Revenue & Customs. HS275 Business Asset Disposal Relief (2026) To qualify when selling all or part of a business you must have been a sole trader or business partner and owned the business for at least two years up to the sale date. For shares, you must have been an employee or office holder and held at least 5% of both the shares and voting rights in a trading company for at least two years.10GOV.UK. Business Asset Disposal Relief
The Rates You Will End Up At
From 6 April 2025, individuals pay 18% on gains falling within the basic rate Income Tax band once added to taxable income, and 24% on the portion above it or if already a higher or additional rate taxpayer. The rates apply equally to all asset types. Trustees and personal representatives of deceased estates pay a flat 24%.11HM Revenue & Customs. Capital Gains Tax Rates and Allowances
The Loss Ordering Rules
Losses come off in a strict order. Current-year losses offset current-year gains first, and this is mandatory even if it wastes your Annual Exempt Amount. Next, you deduct the £3,000 allowance. Only then do you apply losses brought forward from earlier years, and here you get a choice: use just enough to bring remaining gains down to the allowance, preserving the rest for future years. Unused losses carry forward indefinitely once properly reported, but the four-year window to report a loss in the first place is fixed.
Submitting the Form
The SA108 is not standalone. It travels with the SA100.
Online, the capital gains pages are built into HMRC’s Self Assessment service. You tick a box indicating you have capital gains, and the SA108 sections open for you to complete. The software calculates totals and files everything as a single submission, and you get an electronic confirmation with a reference number. Commercial tax software works the same way.
Paper filers download the SA108 pages from GOV.UK, complete them by hand, and post them with the SA100 to:
Self Assessment
HM Revenue & Customs
BX9 1AS
United Kingdom12GOV.UK. Complete Your Self Assessment Tax Return for the Last Tax Year
If you live outside the UK, use HM Revenue & Customs, Benton Park View, Newcastle Upon Tyne, NE98 1ZZ. There is no automatic confirmation for paper returns, so a Post Office certificate of posting is worth having.
UK Residential Property: The 60-Day Rule Comes First
If you sell a UK residential property that produces a taxable gain, you cannot wait until Self Assessment. Since 6 April 2020, UK residents and non-residents must report the disposal and pay the estimated CGT within 60 days of completion.13GOV.UK. Report and Pay Your Capital Gains Tax: If You Sold a Property in the UK UK residents can skip the service if total gains for the year are within the tax-free allowance. Non-residents must report every disposal of UK land or property regardless of the tax due.14Low Incomes Tax Reform Group. Non-Residents and Capital Gains Tax
You still include the disposal on the SA108 at year end, and the tax paid through the 60-day report is credited against the final bill. Missing the 60-day step brings its own late-filing penalties on top of any interest.
Deadlines and Penalties
SA108 deadlines are the SA100 deadlines. Paper: 31 October following the end of the tax year. Online: 31 January. CGT must be paid by 31 January regardless of how you filed.15GOV.UK. Self Assessment Tax Returns: Deadlines
Late filing triggers escalating penalties:16GOV.UK. Self Assessment Tax Returns: Penalties
- Immediately: £100 fixed penalty, even if you owe no tax.
- After 3 months: daily penalties of £10 per day for up to 90 days (maximum £900).
- After 6 months: a further 5% of the tax due or £300, whichever is greater.
- After 12 months: another 5% of the tax due or £300, whichever is greater.
Separate penalties apply for inaccuracies. Errors from a lack of reasonable care attract 0% to 30% of the extra tax; deliberate errors 20% to 70%; deliberate concealment 30% to 100%.17GOV.UK. Penalties: An Overview for Agents and Advisers Telling HMRC about errors, helping quantify the extra tax, and providing access to records all reduce the penalty.
Correcting Mistakes After Filing
You have 12 months from the 31 January filing deadline to amend a return, so a 2024–25 return filed on time can be corrected until 31 January 2027.18GOV.UK. Self Assessment Tax Returns: If You Need to Change Your Return Online filers amend through their HMRC account; paper filers send a corrected return to the same address.
After the 12-month window closes, the only way to recover overpaid tax is an overpayment relief claim, made in writing within four years of the end of the relevant tax year, with a full explanation, revised calculations, and supporting documents. HMRC does not extend that deadline for late discovery or illness.
Records to Keep
HMRC’s minimum for capital gains records is one year after the 31 January deadline for the relevant tax year.19GOV.UK. Capital Gains Tax: What You Pay It On, Rates and Allowances – Records In practice, keep them longer. If HMRC opens an enquiry you need records for the whole process, and if you are carrying forward losses you need to prove the original loss for as long as any of it is still unused. Five years after the filing deadline is a safer working rule, and indefinitely for any losses still on the books.