What will my Self Assessment bill be?
Nine steps through every kind of income a return asks about, ending in the bill, the payments on account most people do not see coming, and the reliefs the answers say you have missed.
Why this exists: ~600,000 paid late · £325m in penalties. Avoids £100+ penalties, finds missed reliefs.
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Rates as at 6 April 2026 — the 2026/27 tax year.
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The rules behind this
Every figure above comes from one of these. Where the tool has interpreted rather than calculated, it says so in the finding.
A return must be filed by 31 January following the tax year, with a £100 penalty for lateness whether or not tax is due.
Payments on account of half the previous year's liability are due where the bill exceeds £1,000.
Finance costs on residential property are relieved as a basic-rate tax reduction, not a deduction.
UK residential property gains must be reported and paid within 60 days of completion.
Notification of chargeability is due by 5 October after the end of the tax year.
Questions people ask
Why is my January bill more than the tax I owe?
Payments on account. Once a bill passes £1,000, HMRC asks for half of it again towards next year at the same time. So the first January is 150% of what you were expecting — which is the single most common reason a return is filed on time and paid late.
Can I reduce the payments on account?
Yes, if you know next year will be lower. But reduce them too far and HMRC charges interest on the shortfall, so it is worth being conservative rather than optimistic.
Where does mortgage interest go?
In the finance costs box, not in expenses. It is not deductible — you get a credit worth 20% of it instead. Putting it in expenses overstates the relief and is the most common landlord error.
Do I have to file if I owe nothing?
If HMRC has issued a notice to file, yes — and the £100 penalty applies regardless of the tax. If you no longer need to file, ask them to withdraw the notice rather than simply not filing.
What if I have missed the deadline?
File as soon as possible. The £100 is fixed, but daily penalties start at three months and add up to £900, so the cost of waiting is far higher than the cost of the first mistake.