The verdict in three sentences
A custom client portal for a London accounting firm costs 28,000 to 68,000 GBP in 2026, depending on the number of modules and how deeply it integrates with your production software. Expect 10 to 16 weeks from scoping to go-live, plus maintenance of 15 to 18 %/year of the initial budget. The return comes from ending insecure email exchanges: 12 hours saved per week per staff member on document collection.
What does each module cost?
A portal is built brick by brick. Here are the 2026 ranges seen on the London market for custom development (excluding subscription SaaS tools).
| Module | Scope | Indicative 2026 cost (GBP) |
|---|---|---|
| Core + authentication | Client accounts, roles, MFA | 7,000 - 12,000 |
| Document vault (DMS) | Upload, filing, versions, search | 6,000 - 12,500 |
| E-signature | Engagement letters, filings | 4,500 - 9,000 |
| Secure messaging | Threads per file, notifications | 4,000 - 8,000 |
| Billing & payment | Invoices, reminders, card payment | 5,000 - 10,000 |
| Client dashboard | Tax deadlines, KPIs | 3,500 - 7,000 |
| Staff back-office | Collection tracking, statuses | 4,500 - 9,000 |
A Starter core (authentication + DMS + messaging) starts around 28,000 GBP. A full portal with signature, payment and a production connector reaches 58,000 to 68,000 GBP.
Integration, hosting and maintenance
Total cost is not limited to initial development. Integration with your production software (Xero, Sage, IRIS, CCH) and secure hosting weigh heavily over time.
| Item | Detail | 2026 order of magnitude |
|---|---|---|
| Production software connector | API or sync gateway | 7,000 - 16,000 GBP |
| Secure EU/UK hosting | Server, backups, certificates | 170 - 500 GBP/month |
| Maintenance | Fixes, evolutions, support | 15 - 18 %/year of budget |
| Annual security audit | Penetration test, updates | 3,000 - 6,000 GBP/year |
| GDPR-compliant hosting | Client data in UK/EU | included in hosting |
For a 50,000 GBP project, maintenance represents 7,500 to 9,000 GBP/year, to be budgeted from the first year.
Mini case study
Mark, partner of a 14-person firm in central London, handled document collection by email and file transfer. Each staff member lost around 12 hours per week chasing and refiling, at an estimated loaded cost of 40 GBP/hour. Across 12 productive staff, that was 12 x 12 x 40 = 5,760 GBP/week of scattered time. A portal at 48,000 GBP (DMS + signature + messaging + payment) that halves this time saves around 2,880 GBP/week, or over 140,000 GBP/year of recovered capacity. The investment pays back in under 4 months in time-equivalent terms, not counting fewer errors and higher client satisfaction.
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FAQ
Is a custom portal more expensive than a SaaS subscription?
Upfront, yes: an accounting SaaS costs 9 to 30 GBP/user/month. But beyond 15 staff and with specific integration needs, custom pays off within 24 to 36 months while making you the owner of the tool.
Is e-signature genuinely compliant?
Yes, provided you use an eIDAS/UK-recognised provider (advanced or qualified level). The signature module relies on such providers; expect 4,500 to 9,000 GBP for integration, plus per-signature fees.
How long until go-live?
Between 10 and 16 weeks: 2 to 3 weeks of scoping and mockups, 6 to 10 weeks of development, 2 to 3 weeks of testing and data migration.
Can the portal connect to our production software?
Yes, via API or gateway. Cost depends on the vendor's openness: from 7,000 GBP for a documented API to 16,000 GBP for a complex multi-flow integration.
What does maintenance cover?
Fixes, security updates, minor evolutions and support. Expect 15 to 18 %/year of the initial budget, adjustable to the service level you require.
Let's scope your project. Tell us your priority modules (DMS, signature, payment), your production software and your headcount, and we'll frame an indicative budget between 28,000 and 68,000 GBP. 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.
