US Tax Return Preparation for Expats: The Dec 15 Letter
US tax return preparation for expats: how one letter posted by October 15 buys a discretionary extension to December 15, and what it never extends. Talk to us.
The letter that buys two months
Americans abroad who cannot file by 15 October may request a further discretionary two-month extension to 15 December. It is requested only by a signed letter postmarked by 15 October, it is granted at the IRS's discretion, and silence is approval: you hear nothing unless you are refused.
This is the least understood deadline in US tax return preparation for expats. It is not a form. It cannot be e-filed. It appears in Publication 54 in two sentences and nowhere on a checklist. And for US citizens living in London, Geneva or Singapore, it is frequently the difference between a considered return and a defensive one. At Jungle Tax we post these letters every October for clients whose UK figures, partnership K-1s or trust statements simply do not exist by the middle of the month.
What exactly is the December 15 extension?
The US extension calendar for a citizen or resident alien living abroad runs in three stages, and most people only know the first two.
- Stage one — the automatic two months to 15 June. If, on the regular due date, you live outside the United States and Puerto Rico and your main place of business or post of duty is outside the United States and Puerto Rico, you get an automatic two-month extension. You claim it by attaching a statement to the return explaining which situation qualified you. No form, no request.
- Stage two — Form 4868 to 15 October. If June is not enough, you file Form 4868 by the extended June due date and tick the box confirming you are out of the country and a US citizen or resident. This carries you to 15 October.
- Stage three — the discretionary two months to 15 December. This is the letter. The IRS instruction in Publication 54, Tax Guide for U.S. Citizens and Resident Aliens Abroad, is to send a letter explaining the reasons you need the additional two months, and to send it by the extended due date. You will not receive any notification unless the request is denied.
Where does the authority come from?
The automatic two months for taxpayers abroad sits in the regulations governing extensions for persons outside the United States. The additional two months is a discretionary extension: the Commissioner may grant a reasonable extension beyond the automatic period, and the practice for filers abroad is the December 15 date described in Publication 54. Some template letters circulating online cite one regulation, some another. The citation is not what carries the request. What carries it is that the letter is timely, signed, identifies you correctly, and states a reason a human reviewer would accept.
Who actually qualifies to ask for it?
Three conditions, all of which must hold.
- You must have qualified for the abroad extension in the first place — living outside the US and Puerto Rico on the regular due date, with your main place of business or post of duty outside the US and Puerto Rico, or on military or naval duty outside the US.
- You must already have a valid extension to 15 October. In practice this means a timely Form 4868. Asking for December when you never extended to October is asking the IRS to forgive a lateness that has already begun.
- You must still be abroad and still genuinely unable to file. The request is discretionary, and the reason matters.
Non-resident filers on Form 1040-NR, and US-resident filers who happen to be travelling, are not in this population. Neither are dual-status filers in the year of arrival unless they meet the abroad test on the due date. This is worth checking properly rather than assuming; the population that qualifies is narrower than the internet suggests.
What must the October 15 letter actually contain?
There is no prescribed form, which is precisely why so many letters are weak. A letter that will survive review contains, in this order:
- Your full legal name exactly as it appears on the return, and your spouse's name if the return will be joint.
- Your Social Security Number or ITIN, and your spouse's, for a joint return.
- Your current foreign address — the same address that will appear on the return.
- The tax year and the form you will file, stated explicitly (for example, "Form 1040 for the calendar year ended 31 December 2025").
- A statement that you previously filed Form 4868 and hold a valid extension to 15 October, with the date it was filed or e-filed and the confirmation reference if you have one.
- A statement that you qualify as a taxpayer abroad, naming the country of residence and the basis (residence abroad, or main place of business or post of duty abroad).
- The specific reason the further two months are needed. Not "more time". A reason.
- An explicit request for a discretionary two-month extension of the time to file to 15 December.
- Your signature and the date. An unsigned letter is a nullity. If a representative signs, a valid Form 2848 must already be on file.
Reasons that work, and reasons that do not
The strongest reasons are structural and verifiable: a foreign employer's payroll statement or foreign pension reporting that is not issued until later in the year; a UK, Irish or Australian tax year that closes on a date incompatible with the US calendar year; late Schedule K-1s from a partnership or S corporation on its own extended deadline; foreign trust statements; a foreign controlled company whose statutory accounts are not finalised; an audit or investigation abroad; serious illness or bereavement; records held in a jurisdiction you cannot currently access.
The weakest reason is the one most commonly written: "I have been busy and need more time." It invites the only outcome you do not want, which is a denial letter received in November with the clock already run.
Where to post it, and how to prove you did
Send the letter to the Austin, Texas service centre that handles returns filed from abroad; the current address is the one printed in the Form 1040 instructions for taxpayers outside the United States, and it should be confirmed against the instructions for the year in question rather than copied from a blog. Use a method that generates a dated receipt. Under the timely-mailing-is-timely-filing rule, a US postmark on or before 15 October is what counts, and a designated private delivery service receipt performs the same function. Ordinary foreign post with no proof of posting leaves you with nothing to show if the IRS later says it never arrived.
Keep three things in the file: a scanned copy of the signed letter, the posting or courier receipt, and a contemporaneous note of the date it was handed over. That package is your reasonable-cause evidence if the position is ever tested.
Silence means yes: how approval actually works
This is the feature that unsettles clients most. The IRS does not send an approval. Publication 54 states plainly that you will not receive any notification unless your request is denied. There is no acknowledgment letter, no transcript code you can rely on, and no telephone line that will confirm it while you wait.
Practically, that means the correct posture is: post the letter early in October, not on the fifteenth, so that a denial has time to reach you; prepare the return as though December 15 is real; and if a denial does arrive, file immediately, because the late-filing exposure runs from 15 October and grows monthly. If you post on the deadline itself and a denial is issued, you may not learn of it until well into the penalty period.
What the December 15 extension does not extend
This is where most published guidance stops far too early. The letter buys time to file the income tax return. It buys nothing else.
| Obligation | Does the December 15 letter extend it? | What actually applies |
|---|---|---|
| Form 1040 filing | Yes, if granted | Moves the filing date from 15 October to 15 December |
| Payment of tax due | No | Tax was due on the original April due date; interest runs from then |
| FinCEN Form 114 (FBAR) | No | FBAR runs on its own automatic extension to 15 October; there is no December equivalent |
| Form 8938, 5471, 8621, 8865, 3520 | Yes, indirectly | These are filed with the return, so they follow the extended return date |
| Form 3520-A (foreign trust) | No | Separate March deadline, separately extended |
| State income tax returns | No | Each state sets its own rules; most do not recognise a federal discretionary letter |
| UK Self Assessment | No | HMRC operates an entirely separate calendar |
Payment, interest and penalties
No extension of time to file has ever been an extension of time to pay. Interest accrues on unpaid tax from the regular due date of the return, and it compounds. The failure-to-pay penalty runs at a monthly rate on the unpaid balance; for filers who qualified for the automatic abroad extension, the late-payment penalty is measured from the extended June payment date rather than April, but interest is not so generous. The failure-to-file penalty is the expensive one, at a far higher monthly rate — and a granted December extension is what keeps it switched off between 15 October and 15 December.
For a high-net-worth filer with material tax due, the correct sequence is therefore: estimate generously, pay before the deadline, and only then worry about the filing date. A payment made with Form 4868 in June, deliberately over-estimated, converts a penalty problem into a refund or a credit forward.
State returns are the quiet failure
A US citizen in the UK who kept a California, New York or Virginia filing obligation does not get to December by writing to Austin. Some states grant an automatic extension that mirrors the federal one; some require their own form; some recognise the federal extension only if no state tax is owed. Anyone still tethered to a high-enforcement state should treat the state calendar as a separate project, and should be reviewing whether the residency tie itself is still defensible. That is a cross-border tax planning question as much as a compliance one.
The cross-border trap: December 15 lands before HMRC's 31 January
Here is the interaction that generalist expat pages handle badly, and it is the single most important point in this guide for a US citizen resident in the UK.
The UK tax year ends on 5 April. The online Self Assessment filing and payment deadline for that year is the following 31 January. So a US citizen filing a US return for the calendar year on 15 December is filing six weeks before the UK return covering the overlapping period is even due, and often before the UK liability has been finalised or paid.
| Milestone | United States (IRS) | United Kingdom (HMRC) |
|---|---|---|
| Tax year | 1 January – 31 December | 6 April – 5 April |
| Standard filing date | 15 April | 31 January (online) |
| Automatic relief for those abroad | Two months, to 15 June | None equivalent |
| Requested extension | Form 4868, to 15 October | No general extension mechanism |
| Final discretionary extension | Letter by 15 October, to 15 December | None |
| Payment date | April due date; interest from then | 31 January, with payments on account 31 January and 31 July |
| Paper alternative | Paper filing available year-round | Paper return due 31 October |
Foreign tax credits when the UK tax has not been paid yet
A December filer claiming foreign tax credits on Form 1116 faces a timing question that an April filer often escapes. Credits are ordinarily claimed on a cash-paid basis, and UK tax for the overlapping year may not be paid until the following 31 January. Electing to claim credits on the accrued basis solves the timing mismatch, but the election is binding for all future years and changes how later UK adjustments must be handled — a redetermination of foreign tax generally requires notifying the IRS, and an under-appreciated consequence is that a routine HMRC amendment two years later can drag an old US year back open.
There is also a currency question. UK tax paid in sterling must be translated, and the convention differs depending on whether you are on the paid or accrued basis. Getting this wrong on a December-filed return is easy, because the sterling figures are provisional at the moment you file.
The strategic case for not using the December extension at all
For many UK-resident Americans, December 15 is the wrong target. If your UK position for the overlapping period will not be settled until the following January, a December US filing is a provisional filing, and provisional filings become amended returns. Two better routes usually exist: pay generously in June and file in October on the best available figures, accepting a small amendment risk; or take the December extension deliberately so that late-arriving K-1s, trust statements or company accounts can be reflected once, correctly, rather than twice. The choice depends on which specific document is missing. Our UK tax services and US tax services teams run both sides of that calendar together, which is the only way the question can honestly be answered.
Form 2350 is a different instrument — do not confuse them
Form 2350 exists for one narrow purpose: a filer who expects to qualify for the foreign earned income exclusion, but only after the return is due, because the bona fide residence test or the physical presence test will not be satisfied until later. It extends the filing date to roughly thirty days beyond the date you can reasonably expect to qualify, and it must be filed by the regular due date of the return.
| December 15 letter | Form 2350 | |
|---|---|---|
| Purpose | Additional time to file, general | Time to qualify for the foreign earned income exclusion |
| Format | Free-form signed letter | Prescribed IRS form |
| Deadline to request | By 15 October | By the regular due date of the return |
| Discretionary? | Yes | Granted where the test is met |
| Typical user | Established expat awaiting documents | Someone who moved abroad mid-year |
Someone who moved from New York to London in the autumn will usually need Form 2350, not the December letter. Someone who has lived in London for nine years and is waiting on a partnership K-1 needs the letter.
The electronic filing window nobody mentions
The IRS closes its individual electronic filing system for annual maintenance in late November each year and reopens it in January. A return filed on 15 December therefore often cannot be e-filed at all, and must be posted on paper. This has consequences a December filer should plan for: paper returns from abroad take months to process, refunds are slow, direct-debit payment options may be unavailable, and the only proof of timely filing is the posting receipt. If your return will carry a refund, a December paper filing may mean waiting well into the following year for it.
Plan the mechanics before you plan the date. Confirm the current-year electronic filing shutdown schedule, arrange payment separately and electronically, and budget for certified or courier posting.
If you have not filed for years, the letter is the wrong tool
A discretionary extension protects a current year. It does nothing about prior years, and requesting one does not put a delinquent filer into good standing. If you have missed US returns, missed FBARs, or unreported UK pensions, ISAs or investment accounts, the relevant machinery is the streamlined filing compliance procedures — specifically the Streamlined Foreign Offshore Procedure for those who meet the non-residency requirement — which addresses several years at once and, for eligible non-willful taxpayers abroad, without the offshore penalty.
Sending a December extension letter for the current year while three earlier years sit unfiled is a common and unhelpful sequence: it draws attention to a filer who is not yet ready to be looked at. The correct order is to scope the historic exposure first, then decide how the current year should be filed so that it is consistent with the disclosure. Our IRS streamlined filing team handles this sequencing daily, and the ordering genuinely matters.
A December 15 checklist for the sophisticated filer
- Early October: confirm the Form 4868 was validly filed and that the abroad qualification holds on the original due date.
- Early October: identify precisely which document is missing and why it will not arrive before 15 October. That sentence is your letter.
- Before 15 October: post the signed letter with proof of posting. Do not leave it to the fifteenth.
- Before 15 October: file the FBAR. It is not covered by the letter.
- Before 15 October: deal with state extensions separately.
- Before 15 October: pay any expected balance. The letter does not extend payment, and interest is already running.
- November: watch for a denial. Nothing arriving is the good outcome.
- Late November: confirm whether the return can still be e-filed, or whether paper is now required.
- By 15 December: file, with the foreign tax credit basis and currency treatment settled deliberately rather than by default.
- January: reconcile the US position against the UK Self Assessment as it is finalised, and record any expected redetermination.
The underlying point
The December 15 extension is a small, obscure, extremely useful mechanism, and it is also a trap for the unwary because it is silent in both directions: you ask by letter and you are answered by silence. Used well, it converts a rushed October return into an accurate December one. Used carelessly — a template letter with a weak reason, posted on the deadline, with the FBAR forgotten and the state return ignored — it produces a false sense of safety and a penalty notice in the spring.
If you are a US citizen or green card holder abroad weighing whether to take the extension this year, or whether your position is complicated enough that October is the safer date, we would be glad to look at it with you. Contact our cross-border team for a confidential consultation. We prepare US and UK returns side by side for founders, executives, fund principals and private clients, and we will tell you plainly whether the letter helps you or simply postpones a problem.
