An API integration connects two software systems so they can share data automatically without manual export and import steps. For accounting practices, API integrations eliminate the double-keying of data between bookkeeping software, practice management tools, payroll systems, e-signature platforms, and HMRC — reducing errors and freeing time that would otherwise be spent on data administration.

This guide explains how accounting software APIs work, which integrations deliver the most value in a typical UK practice, and how to assess and manage API integrations responsibly.

How accounting software APIs work

An API (Application Programming Interface) is a defined set of rules for how two software systems can communicate. When your document capture tool pushes an extracted invoice to Xero via its API, it sends a structured data request: "Create a new draft bill with these fields: supplier name, date, amount, VAT amount, account code." Xero's API receives the request, validates it, and creates the draft bill — without any human copying data from one screen to another.

Modern cloud accounting software — Xero, QuickBooks, Sage, FreeAgent — publish open APIs that allow third-party developers to build integrations. This has created a large ecosystem of connected tools: document capture, payroll, expenses, e-signature, CRM, practice management, and HMRC filing tools all connect to accounting software via published APIs.

From your perspective, you typically do not interact with APIs directly. You connect two applications through their in-app integration settings, grant permissions, and the API connection runs in the background. The management task is knowing what is connected, reviewing what data flows where, and maintaining the connections as software updates.

The highest-value integrations for UK accounting practices

1. Document capture to accounting software

What it does: Dext, AutoEntry, or Hubdoc extract data from client documents and push draft transactions to Xero, QuickBooks, or Sage via API. No manual data entry between capture and bookkeeping software.

Impact: eliminates one of the highest-volume manual tasks in bookkeeping; reduces data entry errors significantly; speeds up month-end and year-end processes.

Setup complexity: low — configured within the document capture tool's integration settings, typically in under an hour.

2. Bank feeds

What it does: accounting software connects directly to client bank accounts via Open Banking APIs (powered by providers including TrueLayer, Yapily, and Plaid) and imports bank transactions automatically. The client authorises the connection and transactions flow in daily without manual export.

Impact: eliminates manual bank statement import; provides near-real-time transaction data; significantly reduces bank reconciliation time.

Setup complexity: low — the client authorises the connection via their online banking in the accounting software setup. Re-authorisation is typically required every 90 days.

Note

Open Banking connections are governed by PSD2 and the UK's Open Banking framework, regulated by the FCA and the Open Banking Implementation Entity. Bank feeds connected via Open Banking do not expose account credentials to the accounting software — the connection is read-only and uses a secure authorisation protocol (OAuth).

3. Payroll to accounting software

What it does: payroll software (BrightPay, Sage Payroll, Xero Payroll, Moneysoft) posts payroll journals to the accounting software via API after each payroll run. Gross pay, employer NI, employee NI, PAYE, pension contributions, and net pay journal to the correct accounts automatically.

Impact: eliminates manual payroll journal entry; ensures payroll data is reflected in management accounts promptly; reduces the risk of journal errors.

Setup complexity: moderate — requires configuration of the payroll nominal mapping, which must be set up correctly to avoid misposting.

4. Practice management to accounting software (billing)

What it does: practice management platforms (Karbon, Iris Practice Management) connect to accounting software via API to sync client records, raise invoices, and reconcile payments automatically.

Impact: eliminates duplicate data entry of client information and billing data; gives real-time debtors position in accounting software.

Setup complexity: moderate — requires mapping client records between systems and configuring invoice templates.

5. E-signature to practice management

What it does: e-signature platforms (Adobe Acrobat Sign, DocuSign, Glide) integrate with practice management systems to send engagement letters for signature, track status, and file signed copies automatically.

Impact: eliminates manual chasing of unsigned engagement letters; provides a complete digital onboarding audit trail; required for HMRC AES-standard client authorisations.

Setup complexity: low to moderate — depends on the specific platforms and the depth of integration available.

6. HMRC MTD API

What it does: MTD-compliant software connects directly to HMRC's API to submit VAT returns, receive MTD obligations, and (under MTD ITSA) submit quarterly updates for income tax.

Impact: eliminates manual VAT return filing on HMRC's portal; mandatory for VAT-registered businesses and sole traders/landlords within the MTD ITSA scope.

Setup complexity: handled by the accounting software — you connect the client's Government Gateway credentials to the software, and submissions are made via the MTD API automatically.

Managing API integrations: security and governance

Each API integration is a connection between two systems that transfers data. This creates security and governance considerations relevant to all AI tools and technology for UK accountants:

  • Principle of least privilege: when setting up an API integration, grant only the permissions the integration needs. A document capture integration needs permission to create draft bills — it does not need permission to delete records or access all client files.
  • Review integrations periodically: integrations accumulate over time. Conduct an annual review of all active API connections for each client and practice tool. Remove integrations that are no longer needed.
  • Monitor for unusual activity: most cloud accounting platforms provide an audit log of API activity. Review this periodically for unexpected data access or unusual integration behaviour.
  • Data Processing Agreements: third-party applications connected via API are data processors. Ensure each connected application has a signed Data Processing Agreement in place.
  • Revoke access when relationships end: when a client moves to another accountant, revoke any API access the previous firm had to the client's accounting software. When a software subscription ends, revoke the API tokens for that tool.

Common integration problems and how to resolve them

Duplicate transactions: if a document capture integration and a bank feed both post the same transaction, you get duplicates. Ensure bank rules are configured to recognise the import source and not create duplicates, and that document capture imports are reconciled against the bank feed rather than posted independently.

Broken connections: bank feed connections and some API integrations require periodic re-authorisation. Set a reminder to re-authorise connections before they expire — most fail silently, meaning data stops flowing without an obvious error.

Nominal code mismatches: when two systems use different chart of accounts structures, integration requires mapping between them. Incorrect mapping leads to misposted journals. Test the mapping thoroughly with sample data before going live.

API rate limits: some APIs have rate limits on how many requests can be made per hour or day. High-volume integrations that hit rate limits will slow down or fail silently. Check rate limits for any integration you build on high-volume data.

Key takeaways

  • API integrations connect accounting software, document capture, payroll, practice management, e-signature, and HMRC MTD tools — eliminating manual data transfer between systems.
  • The highest-value integrations in a typical UK practice are: document capture to accounting software, bank feeds, payroll to accounting software, and HMRC MTD filing.
  • Open Banking bank feeds are governed by PSD2 and the FCA; connections are read-only and use secure OAuth authorisation — no account credentials are shared with the accounting software.
  • Each API integration is a data processor relationship — ensure every connected tool has a signed Data Processing Agreement.
  • Review active integrations annually, apply least-privilege permissions, and revoke access when it is no longer needed.

Frequently asked questions

What is Open Banking and how does it power accounting software bank feeds?

Open Banking is a regulatory framework, implemented through PSD2 in the UK and overseen by the FCA and the Open Banking Implementation Entity, that requires banks to share transaction data with authorised third parties when the account holder consents. Accounting software bank feeds use Open Banking to access client transaction data with the client's authorisation — using secure OAuth rather than sharing banking credentials. The connection is read-only and can be revoked by the account holder at any time through their bank's Open Banking management interface.

Are API integrations secure?

Yes, provided they are properly configured and managed. Modern cloud accounting APIs use OAuth 2.0 for authentication, which means integrations use secure tokens rather than passwords. The main security risks are: granting excessive permissions (mitigated by applying least privilege), failing to revoke access when it is no longer needed (mitigated by regular access reviews), and insecure handling of API keys in custom integrations (relevant mainly for practices building custom integrations rather than using published app-to-app connections).

Do I need technical expertise to set up API integrations?

For the standard app-to-app integrations available in the Xero App Marketplace or QuickBooks App Store, no technical expertise is required. These are configured through user interfaces with step-by-step setup guides. For custom API integrations (building a bespoke connection between two systems without a published integration), developer skills are required. Most accounting practices use published integrations rather than building custom connections.

What happens to existing integrations when a client changes their accounting software?

When a client migrates accounting software, existing integrations connected to the old platform need to be reconfigured for the new platform. Bank feed connections, document capture integrations, and payroll connections will all need to be set up again for the new software. This is a real migration overhead that is worth accounting for in the effort estimate for any platform migration project.

Can API integrations be used to automate HMRC submissions?

Yes — this is exactly what Making Tax Digital achieves. MTD-compliant accounting software uses HMRC's API to submit VAT returns, retrieve obligations, and (under MTD ITSA) submit quarterly income tax updates. The practice retains responsibility for reviewing the data before submission — MTD does not automate away professional oversight, it automates the mechanical filing step once the data has been reviewed and approved.