The verdict in three sentences
A custom client portal for a London accounting firm costs between EUR 12,000 and EUR 35,000 in 2026, depending on the number of modules (vault, DMS, electronic signature, billing). The typical timeline is 8 to 14 weeks and annual maintenance runs EUR 300 to 700 per month. Given the time saved on document exchanges, the return on investment is usually reached in 9 to 15 months.
What each module costs in 2026
An accounting portal is never a monolithic block: it is assembled from bricks. Here are the ranges seen on the London market in 2026 for a custom build (React/Next.js + secure back office), excluding third-party SaaS subscriptions.
| Module | 2026 cost (EUR) | Indicative timeline |
|---|---|---|
| Portal core + strong authentication (MFA) | 4,500 - 8,000 | 2-3 weeks |
| Digital vault (encrypted upload/download) | 3,000 - 6,500 | 2-3 weeks |
| DMS with tree by client and fiscal year | 3,500 - 7,000 | 2-4 weeks |
| Electronic signature (eIDAS integration) | 2,500 - 5,000 | 1-2 weeks |
| Email/SMS notifications + automated reminders | 1,500 - 3,000 | 1 week |
| Tax deadline dashboard | 2,000 - 4,500 | 1-2 weeks |
A 6-person firm targeting the essentials (core + vault + signature) lands around EUR 14,000 to 19,000. A more structured firm adding a full DMS and tax dashboard climbs to EUR 28,000 to 35,000.
Custom or existing SaaS: the real trade-off
Off-the-shelf SaaS (turnkey accounting vaults) start at EUR 8 to 25 per month per client. So the question is not only the purchase price, but control over the brand, the data and the experience.
| Criterion | Custom portal | Existing accounting SaaS |
|---|---|---|
| Upfront cost | EUR 12,000 - 35,000 | EUR 0 - 1,500 (setup) |
| Recurring cost | EUR 300 - 700/month | EUR 8 - 25/month/client |
| Brand customization | Full | Limited |
| Data ownership | Firm (EU hosting) | Vendor |
| Production tool integration | Custom (API) | Depends on connectors |
| Cost at 80 clients / year | ~EUR 5,000/year | ~EUR 15,000 - 24,000/year |
Beyond 60-80 active clients, the custom portal often becomes cheaper over 3 years, while offering a premium image.
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Mini case study
Mark, partner at a 7-person accounting firm in central London. Each staff member spends an average of 40 minutes a day exchanging documents by email and chasing clients. Across 7 people, that is about 4.6 hours/day, or ~1,000 hours/year. By automating upload, reminders and signature via a portal, the firm cuts this time by 55%, i.e. ~550 hours saved. Valued at EUR 45/hour of loaded cost, that is EUR 24,750/year. For a portal billed at EUR 19,000 plus EUR 5,400/year of maintenance, ROI is reached in about 12 months.
FAQ
Is electronic signature really compliant? Yes, provided you integrate an eIDAS provider (advanced or qualified signature). The integration surcharge is EUR 2,500 to 5,000, plus a per-signature cost of EUR 0.50 to 2 depending on volume.
Where is the accounting data hosted? For a UK/EU firm, hosting within the European Union is preferred, ideally with an ISO 27001-certified provider. Expect EUR 40 to 150/month of hosting depending on traffic.
How long until go-live? Between 8 and 14 weeks depending on the number of modules. A core with a vault only can ship in 6 weeks; a full DMS requires more testing.
What maintenance should I budget? EUR 300 to 700/month: security updates, backups, minor improvements and support. It is essential for a tool handling sensitive data.
Can the portal connect to my production software? Yes, if the vendor exposes an API (most do in 2026). The integration typically costs between EUR 3,000 and 8,000 depending on the connector's richness.
Let's scope your project. Tell us your number of clients, the target modules (vault, DMS, signature) and your indicative budget, and we will frame the scope. 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.
