Accounting firms are unusual in this category: you already own accounting software, you use it daily, and you are still probably invoicing badly.
The reason is that practice billing is a different problem from client bookkeeping. Your own invoicing involves work in progress, recurring fixed fees, time recorded against jobs, and — depending on where you practise — client money held in a separate account that must never be touched.
General invoicing software handles none of that well.
What Practice Billing Actually Involves
Work in progress. Time recorded but not yet billed is an asset and a management problem. Firms that do not track WIP discover they have done three months of unbilled work on a client.
Fixed fees on a recurring cycle. Most practice revenue is monthly retainers for bookkeeping, payroll and compliance. These should bill automatically, and drift when they do not.
Job-based billing against a scope. A year-end accounts job has an agreed fee. Time over that fee is either a write-off or a conversation, and you need to see it before you invoice.
Write-offs and write-ups. Recorded deliberately, because the pattern tells you which clients are underpriced.
Client money separation. If you hold money on behalf of clients, it is not your revenue and must sit in a designated client account, reconciled separately. This is a regulatory requirement, not a preference.
The Options
Practice management with billing built in
Karbon — workflow, jobs, client communication and billing. Strong on the work management side, which is where most practice pain actually sits.
Senta or Glide — practice management aimed at UK firms, with deadline tracking, workflows and billing.
Xero Practice Manager — jobs, time, WIP and invoicing, integrated with Xero. The natural choice for Xero-centric firms.
Intuit Practice Management — QuickBooks-centric equivalent.
Ignition — proposals, engagement letters, payment collection and recurring billing in one flow. Worth naming separately because it solves the engagement-to-payment problem specifically, which most firms handle manually.
General accounting software for your own books
Xero or QuickBooks — you probably already run one for the practice. Adequate for simple fixed-fee billing; weak on WIP and job-based billing.
Time and billing specialists
Harvest or Toggl Track — time to invoice, straightforward. Fine for a small practice with simple billing; no practice workflow.
How They Compare
| Tool | WIP tracking | Recurring fees | Practice workflow | Best for |
|---|---|---|---|---|
| Karbon | Yes | Yes | Strong | Growing firms, workflow-led |
| Xero Practice Manager | Yes | Yes | Good | Xero-centric firms |
| Intuit Practice Management | Yes | Yes | Good | QuickBooks-centric firms |
| Senta / Glide | Yes | Yes | Strong | UK compliance-led practices |
| Ignition | No | Strong | Engagement to payment | Proposals and collection |
| Xero / QuickBooks | Limited | Yes | No | Small, simple practices |
| Harvest | Basic | Yes | No | Time-based billing only |
Pricing varies by user count and modules and changes regularly. Verify directly, and price for every fee earner who needs access.
Client Money: Get This Right First
The obligation that outranks every software feature, and the one where mistakes are career-affecting rather than inconvenient.
If your firm holds money belonging to clients — tax payments collected in advance, funds held pending disbursement — that money is not yours and must be handled under your regulator’s client money rules.
The core requirements are broadly consistent across jurisdictions and professional bodies:
- A separate designated client bank account, distinct from practice funds
- No mixing of client and practice money, ever
- Regular reconciliation of the client account, documented
- Records showing what belongs to whom at any point
- Restrictions on withdrawals, including using client money to pay your own fees before properly billing them
Specific rules differ — ICAEW, ACCA and other bodies in the UK each publish client money regulations; US CPA firms face state board requirements; other jurisdictions have their own. Check your own regulator’s current rules, because they are updated and the details matter.
Your invoicing software does not make you compliant. What it must do is not obstruct proper separation — which means being able to distinguish clearly between a fee you have billed and money you are holding.
Engagement Letters and Fee Disputes
Practical, and the thing that prevents most billing arguments.
Every client needs a current engagement letter stating scope, fee basis and what is excluded. Professional bodies generally require this, and it is your only defence in a fee dispute.
Out-of-scope work needs agreeing before it is done. “We also had to sort out the VAT” discovered on an invoice is how relationships end. Tools like Ignition handle scope change as a re-engagement rather than a surprise line item.
Bill promptly. The longer between work and invoice, the more likely it is queried. WIP sitting for months is both a cash flow problem and a dispute risk.
Record write-offs. A client repeatedly written down is underpriced, and you will not see the pattern unless you record it.
Common Mistakes to Avoid
- Not tracking WIP. You discover months of unbilled work at the worst time.
- Mixing client money with practice funds. A regulatory matter, not an accounting preference.
- Using client money to settle your own fees before properly billing. Specifically restricted under most client money rules.
- Out-of-date engagement letters. Your only protection in a fee dispute, and frequently years old.
- Doing out-of-scope work without agreeing it. The most common cause of unpaid invoices in practice.
- Not recording write-offs. Hides which clients are underpriced.
- Billing late. Old invoices get queried more often and paid later.
- Using your practice accounting software as practice management. Xero runs your books; it does not manage jobs, deadlines and WIP.
FAQs
What is the best invoicing software for an accounting firm?
Practice management with billing built in — Karbon, Xero Practice Manager or Senta — because your problem is jobs, WIP and recurring fees rather than producing documents. Ignition specifically if proposals and collection are the pain point.
Can I just use Xero or QuickBooks for practice billing?
For a small practice with simple fixed fees, yes. Once you need WIP tracking, job-based billing and write-off reporting, you want practice management alongside.
What are the rules on holding client money?
Client money must be held in a separate designated account, never mixed with practice funds, reconciled regularly, and withdrawn only under specific conditions. Exact requirements vary by regulator and jurisdiction — check your professional body’s current rules.
How do I reduce fee disputes?
Current engagement letters stating scope and fee basis, agreeing out-of-scope work before doing it, and billing promptly. Most disputes trace to one of those three.
Should I bill fixed fees or hourly?
Both are used widely and this article takes no position on which suits your practice. What matters operationally is that whichever you choose, you track time against the work so you know whether the fee was right.
Do I need practice management software?
Below about three fee earners, often not — accounting software plus discipline works. Above that, deadline tracking and WIP visibility usually justify it, and missed compliance deadlines carry their own risk.
Key Takeaways
- Practice billing is a WIP and job problem, not a document problem.
- Client money rules outrank every software feature — check your regulator’s current requirements.
- Current engagement letters are your only defence in a fee dispute.
- Agree out-of-scope work before doing it, not on the invoice.
- Record write-offs; they show you which clients are underpriced.
Where to Start
Check how much unbilled WIP is sitting in your practice right now. If you cannot produce that figure quickly, that is the capability to buy — and it means practice management rather than better invoicing.
For related decisions, see our guides to accounting software, invoicing software and time tracking software.