SA109 Paper Filing Deadline: 31 October for Americans

For the 2025-26 UK tax year, the SA109 paper filing deadline is 11:59pm on 31 October 2026. HMRC's own free online service will not accept the SA109 residence pages, so a US-connected taxpayer claiming split-year treatment or non-residence must either post a paper return by 31 October or file through commercial software by 31 January 2027.

That single limitation is the reason so many Americans arriving in or leaving London discover, usually in September, that their UK filing problem is not arithmetic. The numbers are knowable. The route is the constraint — and the route quietly moves the deadline forward by three months, into the exact window when the US return that feeds the UK figures is still open on extension.

Why the SA109 is the one page HMRC's own service will not take

The SA100 main return, employment pages, property pages, capital gains pages and foreign pages can all be filed through HMRC's free Self Assessment service. The residence pages cannot. GOV.UK's guidance on sending your Self Assessment return lists filing "if you lived abroad as a non-resident" among the situations where the online service is unavailable, and directs those filers to commercial software or to download the paper forms instead.

This is not a discretionary policy. It is a build limitation in HMRC's consumer-facing product, and it has persisted for well over a decade. HMRC maintains a separate published list of commercial software suppliers for Self Assessment, and only a subset of those products support the residence pages at all.

The practical consequence for a high-net-worth cross-border filer is stark. If your 2025-26 return must carry the SA109 — because you left the UK part-way through the year, arrived part-way through it, are claiming non-residence, are claiming the personal allowance under the double tax treaty, or are making a claim under the new foreign income and gains regime — then you have exactly three routes, and one of them expires on 31 October 2026.

What is the SA109 paper filing deadline for 2025-26?

HMRC's published Self Assessment deadlines for the 2025 to 2026 tax year (6 April 2025 to 5 April 2026) are unambiguous:

Obligation	Deadline	Applies to
Paper tax return, including SA109	11:59pm, 31 October 2026	Anyone filing on paper
Online return via commercial software	11:59pm, 31 January 2027	SA109 filers using approved software
Balancing payment for 2025-26	31 January 2027	Every filer, regardless of route
First payment on account for 2026-27	31 January 2027	Where payments on account apply
Registration for Self Assessment	5 October 2026	New filers for 2025-26

Note what the table does not do: it does not move the payment date. Filing on paper by 31 October does not accelerate your tax payment, and filing by software on 31 January does not delay it. The money is always due on 31 January 2027. Only the filing route moves.

Does registering late change the paper deadline?

Yes, in one specific circumstance. GOV.UK states that if you register for Self Assessment after 5 October 2026, HMRC will issue a letter or email carrying a different filing deadline — three months from the date on that notice. An American who moved to London in, say, February 2026 and only registers in November 2026 may therefore find that their paper deadline sits in early 2027 rather than having already passed. The payment date, again, does not move.

This matters because a great many US arrivers do not realise they have a UK filing obligation at all until an accountant tells them. If you are in that position, do not assume the 31 October date has already defeated you — check the notice.

Why does HMRC set the paper deadline three months earlier?

Because for paper returns HMRC calculates the tax. That calculation service is what the 31 October date protects; HMRC's internal guidance is explicit that it will not guarantee to compute the liability in time where a paper return arrives after the October deadline. The earlier date is administrative, not punitive — but the penalty consequences of missing it are identical to missing any other filing deadline.

The collision: 15 October in Washington, 31 October in London

Here is the problem no generalist SA109 guide addresses. The American filer's US return for calendar year 2025 sits on a different clock, and the two clocks intersect badly.

Stage	United States (IRS)	United Kingdom (HMRC)
Tax year covered	1 January – 31 December 2025	6 April 2025 – 5 April 2026
Standard filing date	15 April 2026	31 January 2027 (online)
Automatic extension for those abroad	15 June 2026	None
Further extension	15 October 2026 (Form 4868)	None
Paper-only supplementary pages	Not applicable	SA109 — 31 October 2026
Tax payment date	15 April 2026 (interest runs from then)	31 January 2027

The IRS confirms on its guidance for US citizens and resident aliens abroad that a calendar-year filer living overseas gets an automatic extension to 15 June, and may extend to 15 October by filing Form 4868. Most of our clients use the full extension, because the UK figures they need for the foreign tax credit are not settled until well into the UK year.

So the sequence for an American in London who wants to file on paper looks like this: the US return is finalised on or around 15 October 2026; the UK paper return, including the SA109, must be signed, posted and physically received by HMRC by 31 October 2026. Sixteen days. Across an ocean. With international post that HMRC does not treat as delivered until it arrives.

And the dependency often runs both ways. A departing American may need the US result to size a treaty position; an arriving American may need the UK split-year date settled before the US return can allocate income correctly. Where the two returns are genuinely interdependent, sixteen days is not a plan.

Which route should a US-connected taxpayer actually choose?

Route one: paper return by 31 October 2026

Appropriate where the picture is simple and settled — a clean departure with a P45, no post-departure UK source income, no complex foreign tax credit interaction. The advantages are that HMRC computes the tax and the file is closed early. The disadvantages are the compressed timetable, the reliance on post, and the absence of any electronic filing receipt.

If you take this route, send it by a tracked service and retain proof. HMRC's receipt date, not your posting date, governs.

Route two: commercial software by 31 January 2027

This is the route we use for the overwhelming majority of US-connected clients, and for one reason: it buys three months of alignment. The US return is finalised in October, the UK figures are then reconciled properly against it, and the SA109 is filed electronically with a timestamped submission receipt. You also gain the ability to amend electronically within the amendment window, which is materially easier than re-papering a return.

The trade-off is that you — or your agent — must be using a product that actually supports the residence pages. Not all recognised Self Assessment software does. Verify this in September, not in January.

Route three: filing through an agent

An agent filing on your behalf uses professional software that carries the SA109 and files to the 31 January deadline. The one item people underestimate is lead time on authorisation: HMRC agent authorisation is not instantaneous, and for a client with no prior UK filing history there is a registration step in front of it. If you are engaging US-UK tax accountants for a 2025-26 residence claim, start the authorisation in the autumn.

Can you file the SA100 online and post the SA109 separately?

No, and this is the single most expensive misunderstanding we see. A Self Assessment return is one document. The supplementary pages form part of it. Filing an online SA100 through HMRC's free service and then posting the SA109 afterwards does not produce a valid return containing a residence claim — it produces an online return that omits the claim, plus loose paper that HMRC has no clean way to associate with it.

The practical result is usually worse than a late filing penalty. HMRC processes the online return as filed, taxes the individual as UK resident for the whole year on worldwide income, and issues a demand. Unwinding that requires an amendment or an overpayment relief claim, and by then interest has begun to run. Choose one route for the entire return.

How do provisional figures work when your US return is not final?

This is the release valve, and it is the reason the 31 October paper route is survivable in a genuine cross-border case. HMRC's own Self Assessment Manual guidance on provisional and estimated figures states that taxpayers should put provisional figures in returns rather than delay submission, provided the figures are reasonable and take account of all available information.

The conditions attached to that concession matter:

• The provisional figure must be a genuine best estimate built on the same basis as the final figure. HMRC's guidance is explicit that there should be no short cuts merely because the number is provisional.
• You must identify which figures are provisional and explain, in the free-text "any other information" space on the return, why a final figure was unavailable and when you expect to supply it.
• You must actually follow up. HMRC states that where final figures are not provided it will take action to obtain them, and in some cases that means opening an enquiry.

Used properly, provisional figures let a departing American file a complete paper return with the SA109 by 31 October 2026 even though a US capital gain, a Schedule K-1, or a foreign tax credit computation remains open. Used lazily — a round number with no explanation — they invite exactly the enquiry you were trying to avoid.

Note also that a provisional figure is not the same thing as an estimated figure you never intend to correct, and it is not a substitute for a claim you have simply not decided on yet. The residence position itself — split-year case, non-residence, treaty claim — should be determined before you file, not estimated.

What the 2025-26 SA109 now asks that earlier versions did not

The form has changed materially, and most competing guidance has not caught up. GOV.UK's publication page is now titled Residence and foreign income and gains (FIG) regime etc (Self Assessment SA109), reflecting the replacement of the remittance basis by the four-year foreign income and gains regime from 6 April 2025.

For an American arriving in London, 2025-26 is therefore the first year in which the residence pages carry the new regime's claim structure rather than the old remittance basis architecture. Two things follow. First, prior-year worked examples circulating online are structurally out of date. Second, the interaction with the US return is different in kind: a FIG claim removes foreign income from the UK charge, which changes what UK tax exists to credit against the US liability, which changes the Form 1116 position. Getting the UK claim right and the US credit wrong is a common and costly outcome.

This is also where the distinction between filing route and filing regime matters. Separate commentary has covered the deferral of Making Tax Digital for Income Tax for taxpayers who need the residence pages; that is a question of which regime you sit in from April 2026 onward. The paper-versus-software question addressed here is about how the 2025-26 return physically reaches HMRC. They are different problems with different dates.

Split-year treatment: the arriving and departing American

Split-year treatment is not optional and it is not elective in the loose sense. Where the statutory conditions for one of the eight split-year cases are met, the treatment applies, and the case must be identified on the SA109. Cases 1 to 3 concern individuals leaving the UK; Cases 4 to 8 concern individuals arriving. Where more than one case is capable of applying, a statutory priority order determines which governs, and that determines the date on which the UK part of the year begins or ends.

That date is the hinge of the entire cross-border year. It determines which employment income is within the UK charge, how a vesting equity award is apportioned, whether a disposal falls in the UK or overseas part, and what the US return can credit. We set the residence conclusion out in detail in our guide to the UK statutory residence test and split-year treatment; the point for present purposes is that the conclusion has to exist before 31 October if you intend to paper-file.

A worked sequence for a US executive who left London in September 2025 illustrates the interaction:

• UK tax year 2025-26 runs to 5 April 2026; the split-year case fixes the UK part as ending in September 2025.
• The US 2025 calendar-year return covers both the UK period and the post-departure period, because US citizenship-based taxation does not stop at the border.
• The foreign tax credit on the US return depends on UK tax attributable to the UK part — a figure that is not final until the UK return is prepared.
• The US return extends to 15 October 2026; the UK paper deadline is 31 October 2026.
• Filing the UK return through software to 31 January 2027 removes the pinch point entirely.

Penalties if the route is missed

Trigger	Consequence
Paper return received after 31 October 2026	£100 fixed penalty, even where no tax is due
Return still outstanding after 3 months	£10 per day, to a maximum of £900
Return still outstanding after 6 months	5% of tax due or £300, whichever is greater
Return still outstanding after 12 months	A further 5% or £300, whichever is greater
Tax unpaid 30 days, 6 months and 12 months after due date	5% of the unpaid tax at each stage, plus interest

The crucial nuance: missing 31 October does not oblige you to accept a penalty. If a paper return has not yet been sent, switching to the software route and filing by 31 January 2027 avoids the late filing penalty altogether, because you have then met the deadline applicable to that route. What you cannot do is post a paper return in December and expect it to be treated as on time.

Where returns for earlier years are also outstanding, the position is more layered and the sequencing of disclosures matters. Our guide to missed UK tax returns and the current filing timetable sets out how to bring multiple years back into compliance in the right order.

The US side of the same file

An American filing an SA109 for 2025-26 has almost always triggered US reporting alongside it, and the two are usually discovered together. A UK bank account opened on arrival, a workplace pension, a stocks and shares ISA, or a UK investment platform will engage FBAR reporting and, above the relevant thresholds, Form 8938. The FBAR carries its own automatic extension to October, which is why it so often lands in the same fortnight as everything else.

Where those US filings were never made — the classic profile being the accidental American who has lived in London for years, or the executive who assumed UK payroll withholding was the end of the matter — the correct response is a structured catch-up rather than a quiet forward filing. The IRS Streamlined Foreign Offshore Procedure exists precisely for non-wilful cases, and it is generally penalty-free for qualifying taxpayers. We handle these through our IRS streamlined filing practice, and we sequence the UK and US filings deliberately so that the residence position stated to HMRC and the residence position stated to the IRS are the same position.

Inconsistency between the two is what draws attention. A split-year date claimed on the SA109 that does not match the allocation used on the US return is visible, and under information exchange it is increasingly visible in both directions.

A practical timetable to 31 October 2026

• Now: confirm whether the 2025-26 return requires the SA109 at all. Non-residence, split-year, a treaty personal allowance claim, or a FIG claim all require it.
• Now: confirm registration status and that a Unique Taxpayer Reference exists. Registration was due by 5 October 2026 for new 2025-26 filers.
• Early September: decide the route. If you intend to paper-file, work backwards from a mid-October posting date, not from 31 October.
• Early September: if using an agent, complete authorisation. This is the step that most often derails the October route.
• Mid-October: finalise the US return, or fix the provisional figures you will use and draft the explanation for the return's free-text section.
• Late October: post by tracked service, or — far more often the better answer — file electronically in the new year with the US position settled.

The judgement in almost every US-connected case comes down to one question: is anything on this return dependent on a US figure that will not exist before mid-October? If the answer is yes, the software route to 31 January is not a convenience. It is the correct professional answer, and the paper deadline becomes irrelevant.

Speak to us before the route decides itself

Jungle Tax prepares US and UK returns for internationally mobile executives, founders and private clients on both sides of the Atlantic. We prepare the SA109 alongside the US return as a single co-ordinated file, so the residence position, the split-year date and the foreign tax credit all agree — and so the filing route is chosen deliberately rather than discovered in late October. If your 2025-26 return carries residence pages, or you have US filings outstanding behind them, contact our cross-border team for a confidential consultation. You can also review our full UK tax services and cross-border compliance support.