HMRC letter codes · One-to-many letters
AIA3MMLetter about Annual Investment Allowance claims and groups of companies
Published by its source on 20 February 2024. A date in the letter that has passed stays passed: what a passed date means.
Last reviewed 14 October 2026 · How Tax Letters checks its facts
In short
HMRC's letter AIA3MM is headed “Annual Investment Allowance claims and groups of companies”.
“AIA3MM … Annual Investment Allowance claims and groups of companies”
CIOT: copy of HMRC letter AIA3MM, published on its page · checked 14 October 2026
Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about AIA3MM. Each fact has its source and the day it was checked beside it.
When HMRC sends it
CIOT said, on its page for HMRC's letter AIA3MM, that HMRC had started a One to Many letter campaign directed at companies within the Mid-Sized Business population encouraging them to review their group Annual Investment Allowance (AIA) claims to ensure they did not exceed the annual allowance.
“HMRC have advised us that they have commenced a One to Many letter campaign directed at companies within the Mid-Sized Business population encouraging them to review their group Annual Investment Allowance (AIA) claims to ensure they do not exceed the annual allowance.”
CIOT: HMRC One to Many letter – Annual investment Allowance Group Claims · checked 14 October 2026
HMRC's letter AIA3MM said HMRC was contacting customers to clarify the rules surrounding Annual Investment Allowance (AIA) and ensure they were fully aware of their tax obligations.
“HMRC are contacting customers to clarify the rules surrounding Annual Investment Allowance (AIA) and ensure they’re fully aware of their tax obligations.”
CIOT: copy of HMRC letter AIA3MM, published on its page · checked 14 October 2026
Time limits and rules
HMRC's letter AIA3MM asks the reader, if they think there is a mistake on their return or on the return of another company in the group, to amend that return within 30 days of the date of the letter.
“If you think there’s a mistake on your tax return, or the return of another company within the group, please amend that return within 30 days of the date of this letter.”
CIOT: copy of HMRC letter AIA3MM, published on its page · checked 14 October 2026
Free help and HMRC contacts
HMRC's letter AIA3MM points to www.gov.uk, search “CA23088”, for guidance about group AIA claims.
“For guidance about group AIA claims, go to www.gov.uk and search ‘CA23088’.”
CIOT: copy of HMRC letter AIA3MM, published on its page · checked 14 October 2026
HMRC's letter AIA3MM says a reader can amend their return online by going to www.gov.uk and searching “Company returns”.
“To amend your return, you can: … amend it yourself online – go to www.gov.uk and search ‘Company returns’.”
CIOT: copy of HMRC letter AIA3MM, published on its page · checked 14 October 2026
Free help first
These routes are described by GOV.UK, TaxAid and Citizens Advice themselves.
- HMRC's extra supportHMRC offers extra support if your health condition or personal circumstances make it difficult to contact HMRC, for example a mental health condition, financial hardship or being treated in hospital.
- Someone to deal with HMRC for youYou can authorise a friend, relative or voluntary organisation to talk to HMRC for you and help you fill in forms.
- TaxAidTaxAid helps people with tax problems who cannot pay for advice. Tax Help for Older People merged with TaxAid in July 2025.
- Citizens AdviceCitizens Advice gives confidential advice online, over the phone and in person, for free.
- CIOT: copy of HMRC letter AIA3MM, published on its page (checked 14 Oct 2026)
- CIOT: HMRC One to Many letter – Annual investment Allowance Group Claims (checked 14 Oct 2026)
- GOV.UK: Get help from HMRC if you need extra support (checked 7 Oct 2026)
- GOV.UK: Appoint someone to deal with HMRC on your behalf (checked 7 Oct 2026)
- TaxAid: About us (checked 8 Oct 2026)
- Citizens Advice: About us (checked 7 Oct 2026)