The verdict in three sentences
For an accounting firm of 20 to 40 staff in London, custom practice software between EUR 25,000 and 60,000 (about GBP 21,000 to 51,000) pays for itself in under 12 months if it targets document collection and engagement tracking. It does not replace your production tool (Xero Practice Manager, IRIS, CCH, Pennylane), it plugs into it through APIs to remove re-keying and email chasing. The 2026 wave of digital tax obligations, with Making Tax Digital for Income Tax starting in April 2026 in the UK and e-invoicing in France from September 2026, is the right moment to start, because your SME clients will ask you to guide them.
Why off-the-shelf tools leave a gap in your organisation
Most London firms already run a strong production tool. The problem sits around it: documents arrive by email, WhatsApp or file transfer, engagement tracking lives in a shared spreadsheet, and time is logged at month end, from memory. The result is under-billed engagement letters and staff who spend their Mondays chasing clients.
| Firm need | Typical market tool | Limit observed in 2026 | What custom software adds |
|---|---|---|---|
| Document collection | Email, generic DMS | 30 to 40% of documents arrive late | Client portal with a checklist per engagement and automatic reminders |
| Engagement tracking | Shared spreadsheet | No view by tax deadline | Dashboard by client, deadline and staff member |
| Time tracking | Manual monthly entry | 10 to 15% of hours never logged | Two-click entry from the engagement record |
| Fee billing | Manual export to accounts | Gap between engagement letter and actual work | Alert once 80% of the hour budget is used |
| Digital tax compliance | Approved platform only | No status tracking per client | Compliance status for each SME client |
| Production integration | Re-keying | 1 to 2 hours per week per staff member | API connector to Xero, IRIS, CCH or Pennylane |
How much does custom accounting practice software cost in 2026
The ranges below are a 2026 order of magnitude for a firm of 15 to 50 staff. They include design, development, testing, go-live and training.
| Scope | Features | Budget (excl. VAT) | Timeline |
|---|---|---|---|
| Collection core | Client portal, document checklist, reminders | EUR 25,000 to 32,000 | 8 to 10 weeks |
| Core + engagements | Deadline tracking, assignment, dashboard | EUR 32,000 to 42,000 | 10 to 12 weeks |
| Engagements + time | Time entry, hour budgets, alerts | EUR 40,000 to 50,000 | 12 to 14 weeks |
| Full + integrations | Production API, fee billing, digital tax status | EUR 50,000 to 60,000 | 14 to 18 weeks |
| Annual maintenance | UK or EU hosting, fixes, minor improvements | 15 to 20% of build per year | Ongoing |
| Mobile option | Photo document upload app for clients | EUR 6,000 to 10,000 | 3 to 4 weeks |
For comparison, a market practice management subscription costs EUR 25 to 60 per user per month, so EUR 7,500 to 18,000 per year for 25 people, with no fit to your processes and no code ownership.
2026 digital tax deadlines: what the software must anticipate
From April 2026, UK sole traders and landlords above GBP 50,000 of income must file quarterly under Making Tax Digital, with the threshold dropping to GBP 30,000 in 2027. Firms with French clients face the September 2026 e-invoicing reception obligation. In both cases your firm becomes the first point of contact for hundreds of clients. Custom software can track, client by client, the software chosen, the onboarding status and any anomalies. It is also a billable advisory service, EUR 300 to 1,200 per client depending on size.
Mini case study
Claire, partner at a 25-person accounting firm in London, measures 4 hours lost per staff member per week on chasing and re-keying. Over 45 working weeks: 25 x 4 x 45 = 4,500 hours. At a loaded cost of EUR 55 per hour, that is EUR 247,500 a year. Recovering only half that time with EUR 48,000 software yields an annual gain of EUR 123,750, a payback in under 5 months. Add 8% more billed hours thanks to budget alerts and the firm also covers annual maintenance (about EUR 8,000).
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FAQ
Do we need to leave Xero or IRIS to go custom?
No. The software connects to your production tool through an API or structured export. You keep your production and add the collection and tracking layer, for EUR 25,000 to 60,000 depending on scope.
How long before go-live?
Allow 8 to 10 weeks for the collection portal alone and 14 to 18 weeks for the full version. Starting in summer means you are ready before the January self-assessment peak.
Is client data compliant with UK GDPR and professional confidentiality?
Yes if hosting is in the UK or EU, with encryption, access logs and two-factor authentication. This security level adds roughly EUR 3,000 to 5,000 to the initial budget.
How many clients make custom software worthwhile?
Above 300 client files and 15 staff, payback usually falls under 12 months. Below that, a market tool at EUR 30 per user per month is often the more sensible choice.
Does the firm own the code?
Yes, the transfer of rights is written into the contract. You can evolve the tool with any provider, unlike a SaaS subscription.
Let's scope your project. We define the scope with you (collection, engagements, time, integrations), an indicative budget between EUR 25,000 and 60,000 and a schedule of 8 to 18 weeks. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
