UK SaaS accounting reference date 2026: how to pick yours, when to change with Form AA01, and the calendar trap that costs solo founders thousands

When you incorporated your UK SaaS at Companies House, the system gave you an accounting reference date (ARD) by default. For most solo founders, that default is the last day of the month they incorporated. If you formed the company on 14 March, your default ARD is 31 March. If you formed it on 27 September, your default ARD is 30 September.

Most founders ignore this entirely until 21 months later when their first set of accounts is due, by which point they're trying to fit a year-end, a Self Assessment, a Personal Tax Return, and Stripe revenue projections into a single panicked weekend.

That's the trap. The default ARD is rarely the right ARD for a UK indie SaaS. There is a 5-year cooldown rule on changes, an 18-month maximum extension, and the wrong choice leaves you filing CT600s, CS01s, PAYE, VAT, and your personal SA all on different cycles for the life of the company.

This is the founder-side walkthrough. Pick the right ARD on incorporation; change it cleanly with Form AA01 within the first 12 months if you got it wrong; align everything to the cycle that minimises admin pain.

The default is wrong for most solo founders

Companies House sets your first ARD to the last day of the month you incorporated. So if you formed your limited company on 12 May 2026, your default ARD is 31 May 2027 for your first accounts.

Three problems with the default:

1. Misalignment with HMRC corporation tax. CT is due 9 months and 1 day after your ARD. A 31 May ARD means CT is due 1 March. Your accounts are due 9 months after that = 28 February. The combined deadline rush sits awkwardly between your personal SA (31 January) and the start of the new tax year (6 April).
2. Misalignment with personal tax (sole-director companies). As a director you'll likely take a small salary + dividends. Dividends are taxed in the tax year they're declared (5 April year-end). If your ARD is 31 May, you're constantly mapping a non-tax-year company year to a tax-year personal return. Double-bookkeeping.
3. Pre-revenue stage trap. Most SaaS founders incorporate in stage 1 (pre-revenue) and start trading in stage 3 (first paying customers). The 9-and-1 CT deadline is set on the ARD, not on the trading start date. A May 2026 incorporation with first paying customers in November 2026 still files year-1 accounts by 28 February 2028 — and HMRC still expects a CT600 filed for the dormant phase if you let the trading start go uncommunicated.

The 4 ARD options for UK SaaS founders

Option	ARD	When CT is due	Why a SaaS founder picks this
Tax year aligned	31 March (or 5 April)	1 January (or 6 January)	Maps directly to HMRC tax year. Personal SA + company year share data. Cleanest for sole-director SaaS.
Calendar year	31 December	1 October	Matches accounting software defaults, US/EU client invoices, and Stripe annual reports. Convenient if half your customers run on calendar year.
Mid-year breather	31 July or 31 August	1 May or 1 June	Avoids January / December crunch. Useful if your founder is a parent or runs a second business with seasonal load elsewhere.
Default (do nothing)	Last day of incorporation month	9m+1d after	Fine if you happen to incorporate at the end of March or December. Otherwise — change it.

Recommended default for solo SaaS founders: 31 March. It maps to the HMRC tax year, simplifies dividend declarations, and aligns CT600 + Self Assessment data so you only have to gather your figures once.

When you can change the ARD — and when you can't

The rules:

• You can shorten the accounting period to any date you like, as often as you want (including multiple times per year — though there's no reason to).
• You can extend the accounting period by up to 18 months from the start of the current period.
• You can only extend once every 5 years (with limited exceptions: insolvency, parent company alignment, court order).
• You cannot change the ARD if your accounts are already overdue.
• You cannot change the ARD to be more than 18 months from the start of the period.

Practical implication: if you incorporated in May 2026 with a default ARD of 31 May 2027, you can shorten the first period to end on 31 March 2027 (giving you a 10-month first period) or extend it to 31 March 2028 (giving you a 22-month first period — wait, that exceeds 18 months, so it's capped at 30 November 2027 = 18 months from 1 June 2026 incorporation = max).

The cleanest move for a May-incorporated company aiming for a 31 March ARD is to shorten the first period to end 31 March 2027. You file a short-period return covering 12 May 2026 to 31 March 2027 (~10.5 months), and from 1 April 2027 onwards you're on annual 12-month cycles.

Form AA01: the 10-minute walkthrough

Form AA01 is filed online via Companies House WebFiling. It is free (no fee) and processed within 24 hours in most cases.

Step 1 — Confirm you're allowed to change

Five-year rule check:

Question	Answer required
Is this your first ARD change since incorporation?	Yes → proceed. No → check date of last change.
If not first: was the last ARD change more than 5 years ago?	Yes → proceed. No → wait or use exception.
Are your accounts currently overdue?	No → proceed. Yes → file the overdue accounts first.
Are you extending or shortening?	Shortening = no 5-year rule. Extending = 5-year rule applies.

Step 2 — Decide on the new ARD

Four common patterns for a UK SaaS founder:

Current default	Recommended target	Action	Result
31 May (May-incorporation)	31 March	Shorten first period	First accounts cover 10.5 months
30 September (Sept-incorporation)	31 March	Shorten first period	First accounts cover 6.5 months
31 December (Dec-incorporation)	31 March	Shorten first period	First accounts cover 3 months
31 January (Jan-incorporation)	31 March	Shorten first period	First accounts cover 2 months

Note: a very short first accounting period (less than 6 months) is unusual but allowed. HMRC will charge CT only on profit earned in that short period. For a pre-revenue SaaS, this is almost always fine.

Step 3 — File AA01 via WebFiling

1. Log into Companies House WebFiling (https://ewf.companieshouse.gov.uk/).
2. Select your company.
3. Select "File a form" → "Change accounting reference date (AA01)".
4. Enter the new ARD.
5. Confirm the period being shortened or extended.
6. Submit.

Processing: usually completed within 24 hours. You'll receive an email confirmation. The new ARD is then visible on your company's public Companies House profile.

Step 4 — Tell HMRC (separately)

This is the bit founders forget. Companies House does not automatically tell HMRC about the ARD change. You need to inform HMRC separately so the corporation tax return deadline is updated.

To inform HMRC:

1. Log into HMRC online services (the Government Gateway account you used to register for CT).
2. Navigate to "Corporation Tax" → "Tell HMRC about a change".
3. Update the accounting period dates.

Or: write to Corporation Tax Services, HM Revenue and Customs, BX9 1AX with your company name, UTR, old ARD, and new ARD.

If you don't tell HMRC, you'll receive a CT return notice based on the old ARD and risk a late-filing penalty when you file based on the new one.

When SaaS founders should consider a non-31-March ARD

The 31 March recommendation isn't universal. Three scenarios where a different ARD is better:

Scenario A — VC-funded with parent company alignment

If your SaaS has taken VC funding through a parent or holding company structure, the ARD should align with the parent's. Most US-led VCs run on a 31 December calendar year. UK companies in their portfolio usually align to the same date for consolidated reporting.

This is also the rare case where you can extend the ARD without the 5-year rule applying — Companies House allows ARD changes specifically to align with a parent company.

Scenario B — Predominantly enterprise / annual contract revenue

If your SaaS has an annual-contract-heavy revenue mix (typically B2B ACV-based), the rev-rec rhythm matters. A calendar year ARD (31 December) often makes the deferred revenue calculation cleaner because annual contracts typically renew on calendar boundaries.

Scenario C — Founder also runs a non-SaaS business

If you also operate a second business (consulting, e-commerce, etc.) and you want unified personal cash-flow planning, align the SaaS ARD to whichever cycle gives you the smoothest combined picture. For most this is still 31 March; for retail-heavy founders with peak Q4, it might be 31 January (post-Christmas wind-down).

The £8,000 calendar trap (and how to avoid it)

A pattern we see often: a founder incorporates in April, sets the default ARD (30 April), starts trading in November of the same year, and doesn't think about Companies House again until the following autumn.

What happens:

• Year 1 accounting period: 1 May 2026 to 30 April 2027 (12 months, ~5 months trading).
• Companies House first accounts due: 31 January 2028 (21 months after incorporation).
• HMRC CT600 due: 1 February 2028 (9 months and 1 day after ARD).
• Personal Self Assessment for tax year 2026/27 (covering dividends taken in 2026/27): due 31 January 2028.

All three deadlines on the same day. With a director-loan reconciliation outstanding because the founder paid themselves £8,000 from the company in October 2026 ahead of dividend declaration, didn't repay it before the CT600 was due, and consequently triggered s455 corporation tax of £2,860 (35.75% of £8,000) plus a beneficial loan charge.

That's the £8,000 calendar trap. The fix is preventative: pick the right ARD on day 1 (or change with AA01 within the first 12 months), and run the year-end + tax-year + personal SA on the same cycle.

Cross-link: the four UK SaaS founder filings

Filing	Frequency	When
CS01 (confirmation statement)	Annual	12 months after incorporation, then yearly
Annual accounts	Annual	9 months after ARD (first set: 21 months after incorporation)
CT600 (corporation tax)	Annual	12 months after end of accounting period; tax due 9m+1d after
PAYE / RTI	Per pay period	Monthly or weekly when staff or director's salary is paid
VAT (if registered)	Quarterly	One month and 7 days after end of VAT period
PSC / officer changes	As they happen	Within 14 days
Self Assessment (director personal)	Annual	31 January following tax year end

A 31 March ARD synchronises CT, accounts, and personal SA into the cleanest possible cycle: file SA in January, file CT and accounts at the end of December (9-month rolling), and CS01 on a separate fixed date that doesn't move.

5 mistakes UK SaaS founders make with their ARD

1. Sticking with the default month-end ARD because Companies House set it. Default is rarely optimal. Change in year 1 with AA01 (no fee, 5-minute filing).
2. Telling Companies House about the change but not HMRC. HMRC operates independently. CT return deadlines are based on the HMRC-recorded accounting period — update separately.
3. Trying to extend the ARD outside the 5-year rule. You'll get rejected. Shortening (instead of extending) avoids the rule.
4. Forgetting the dormant year-1 CT600 obligation. Even if pre-revenue, HMRC expects either a CT600 marked dormant, or notification that the company is dormant (via the dormant company filing CIC34 or by writing to HMRC). Silence triggers automatic late-filing penalties.
5. Changing ARD just before a funding round / sale. Acquirers prefer clean, predictable filing cycles. A last-minute ARD change creates a messy short period and complicates due diligence. Lock the ARD in early; don't fiddle in late stage.

30-min ship-it: align your ARD today

Step 1 (5 min): Decide the target ARD
  - Default for solo SaaS: 31 March
  - VC-backed with US parent: 31 December
  - Heavy annual contract / Q4: 31 January

Step 2 (5 min): Check eligibility
  - First ARD change? Yes → no 5-year rule
  - Accounts overdue? No → proceed
  - Shortening or extending? Shortening preferred

Step 3 (5 min): File AA01 via Companies House WebFiling
  - Log in at ewf.companieshouse.gov.uk
  - "File a form" → "Change accounting reference date (AA01)"
  - Enter new ARD, submit (free, no fee)

Step 4 (5 min): Tell HMRC about the change
  - Log into HMRC online services
  - Corporation Tax → "Tell HMRC about a change"
  - Update accounting period

Step 5 (5 min): Update bookkeeping software
  - FreeAgent, Xero, QuickBooks, Pandle: update company year-end
  - Refresh CT scheduling, dividend declaration timeline

Step 6 (5 min): Add to founder calendar
  - 9 months after new ARD: CT due
  - 9 months after new ARD: accounts due (after first short period rolls)
  - 31 January following: personal SA due

Total time: 30 minutes. Cost: £0. Saves: countless hours of misaligned filing pain.

Cross-link to other UK SaaS founder reading

• UK SaaS Companies House CS01 walkthrough 2026 — the other annual Companies House filing
• UK indie hacker founder taxes 2026 — director's loan, dividends, MTD ITSA
• UK SaaS first hire 2026 — when you bring on your first developer

Faqs

(See FAQs below.)

CTA — get the latest free report

If you're at the stage where ARD planning matters, you've also crossed the threshold of needing to think about whether your SaaS idea has the data to support the next 12-24 months of building. Our latest free report digs deep on a UK SaaS opportunity with real keyword data, competitor analysis, and a full builder prompt — read it free at ideastack.co/reports.