The Submission section takes the return from prepared to filed: pre-flight checks, HMRC’s own validation rules, attachments, the Government Gateway login, the declaration, the submission itself, and a history of every attempt.
Pre-flight checks
Every check must pass before the Submit button unlocks, and every one is re-verified on the server at the moment of filing:
- the year is open for submission (not already accepted, unless amending)
- the client has a valid UTR on their record (checked against HMRC’s check-digit algorithm, not just for presence) — with a link to open the record
- a National Insurance number is entered in Personal Details
- no hidden pages hold figures — a page switched off while it still held entries would file them; the Choose pages link opens the window to show it again or clear it
- calculation parameters exist for the tax year
- there are no blocking calculation warnings
- where the return has a Capital Gains page, the computation is provided — in box 54 or as a PDF attachment — because HMRC rejects an SA108 without one
- a Government Gateway login is saved for the SA100 (see below)
- the return passes HMRC’s validation rules (see below)
Anything failing shows in red with what to fix. Where the fix is on another section of the return, a Go to link beside the check takes you straight there — to the very box where the check names one; come back to Submission afterwards and the list is checked again, or press Re-check to run it at any time.
HMRC’s own rules, run before you file
The last check builds the return exactly as it would be sent and runs HMRC’s own validation rules over it — the schema that fixes what each box may hold, and the business rules HMRC’s gateway applies to the figures. When any fail, the check expands into HMRC would reject this return with one line per broken rule: the box it concerns as a badge, HMRC’s rule in plain words (a zero in a box that cannot be nil, a date outside the year, a cap exceeded, a required box left empty), and a Go to link that lands on that box. The page-level warnings described in how the pages work catch most of these as you type; this check is the safety net that catches the rest and the rules that span pages.
Attachments
PDFs added in the Attachments card are sent to HMRC inside the return — a capital gains computation in place of box 54, for example. PDF only, up to 5 MB in total. Attachments are locked with the return once it is submitted.
Check the return first
Above the Gateway card, a short card offers the whole return on HMRC’s forms: Preview return opens the clickable Return Preview and Download return PDF saves it. Firms with client review switched on see the Client review card here too.
The Government Gateway login for the SA100
HMRC only accepts an SA100 from a Government Gateway account that is allowed to file Self Assessment for that person, so the return is sent with a login. Tax Optimiser uses, in this order:
- The client’s own login, when you have saved one in the Government Gateway login for the SA100 box on the Submission section. Use this only for a client who files under their own Government Gateway account — and if you are an individual filing your own return, this is where you save your login.
- Your firm’s agent Self Assessment login, saved once under your firm’s settings, HMRC tab (Self Assessment Sender Id and Self Assessment Password). This is the usual choice: it covers every client you are authorised to act for, and nothing needs saving against the individual.
The box tells you which login the return will go out under. Logins are stored encrypted and a saved password is never shown again — leave it blank to keep it, type a new one to replace it, or use Remove saved login to delete it.
This login is for the SA100 only. Making Tax Digital for Income Tax does not use it — MTD connects through HMRC’s own sign-in — which is why the box appears here and nowhere on the Making Tax Digital screens. Do not save a Government Gateway password for a client who only uses Making Tax Digital.
Declaration and submit
Tick the declaration — the standard Self Assessment wording that the return is correct and complete to the best of the taxpayer’s knowledge — and press Submit to HMRC. Tax Optimiser builds the return in HMRC’s required format, signs it with the tamper-evident IRmark, sends it, and polls HMRC for the response, all in one step. The section shows a progress panel while this happens — it can take a minute or two — and then the outcome:
- Return accepted by HMRC — the year moves to Accepted and locks against accidental edits.
- HMRC is still processing the return — HMRC has the return but had not answered after a couple of minutes of polling. The year is locked while it waits, and its status is checked again automatically every fifteen minutes; Check status on the history row asks straight away.
- There was a problem submitting the return — HMRC’s errors are listed one per line. Where HMRC’s calculation disagrees with the filed figure, the panel shows both amounts side by side (Filed £a · HMRC calculated £b). The year reopens for correction as soon as you edit it.
The submission history
Every attempt is recorded with its date, status (Waiting for HMRC, Accepted or Rejected), whether it was an amended return, and its HMRC correlation ID — the reference to quote if you ever need to speak to HMRC (or to support@taxoptimiser.co.uk) about a filing. Details on each row expands the IRmark, the sent and accepted times, and every message HMRC returned. If a submission is still Waiting for HMRC, Check status asks HMRC for the outcome again without resubmitting anything; Tax Optimiser also chases stuck submissions automatically in the background every fifteen minutes, so a return interrupted mid-poll usually resolves itself without any action from you.
Next: Amending an accepted return.
