Submit the Final Declaration to HMRC via the Making Tax Digital for Income Tax API to complete a customer's Self Assessment tax year

Steps

1. Confirm all required quarterly updates, annual adjustments, and any other income sources for the tax year have been submitted before attempting the Final Declaration — HMRC removed the separate End of Period Statement step, so software must handle any adjustments that used to go through it directly.
2. Retrieve the customer's outstanding Self Assessment obligations via the relevant obligations endpoint to confirm the final declaration obligation is open for the tax year.
3. Call the Submit a Self Assessment Final Declaration endpoint once the customer confirms they believe all required income information has been submitted and is accurate.
4. Do not treat the declaration as instantly reflected — allow up to an hour for HMRC to mark the obligation as fulfilled before querying its status again.
5. If the customer needs to change previously submitted data after declaring, follow HMRC's documented process for amendments after a final declaration rather than resubmitting the declaration itself.
6. Surface any penalty or late-submission warnings from the obligations/liabilities data to the customer, since Self Assessment penalty rules apply to the Final Declaration deadline.