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Do not build this. Bank feeds and real double-entry books are the product, and both are harder than they look.

Feeds need an aggregator with regulatory permission and bank contracts, which you cannot arrange as an individual. Double-entry accounting is not a table of transactions — it is a ledger where every entry balances, where a period can be closed, and where an accountant can follow what happened. Getting it approximately right produces books nobody can file from.

WHAT TO DO INSTEAD
- Keep it. This one is largely free, which makes it among the weakest cases for building anything
- If you need invoicing rather than accounting, that is a different and much smaller problem — under Invoice Ninja elsewhere in this catalogue

WHAT YOU CAN BUILD, AND SHOULD

**The invoicing half**, where the rules are strict but knowable:
- **Gapless sequential numbering**, per business, per document kind, per year, allocated inside the transaction that issues the document. In much of Europe this is a legal requirement, not a preference
- An issued invoice is never edited or deleted. A mistake is voided with a reason and corrected by a credit note that references it
- Per-line tax rates with totals broken out by rate, which most authorities require, and the rate copied onto the line at issue so a later change never alters an issued document
- Reverse charge for cross-border business customers, with the required sentence printed and both registration numbers shown
- Amounts in integer cents with a currency. Never a float
- The PDF generated once at issue and stored with its hash

**And the import path**, since you cannot have a feed:
- Bank exports as CSV or OFX, with a mapping step, a dry run, and duplicate detection by a stable identifier then by date, amount and payee. A double-imported month is the classic disaster
- Reconciliation against the bank's stated balance, which is the only thing that tells you the data is right

THE SENTENCE TO KEEP
Bookkeeping rules differ by country and change. Write which assumptions you made in the README, and check them with a local accountant before filing anything.

THE ONE-LINE VERSION
It is free and it is real accounting. Build the invoicing if you need it your way — and get the numbering right, because that part is law.