The verdict in three sentences
Custom practice management software for a London accounting firm costs 30,000 to 90,000 EUR in 2026, over a 12 to 24 week build. Market suites remain unbeatable below 10 staff at 40-120 EUR per staff member per month. Custom pays off beyond 15 staff, or when the client portal and ledger integrations become a competitive edge.
What a firm software actually covers
A firm is not just bookkeeping production. The typical functional scope of a 2026 custom project blends production, client relationship and compliance. Here are the modules and their indicative budget weight (2026 order of magnitude).
| Module | Key function | Budget share | Indicative cost (EUR) |
|---|---|---|---|
| Bookkeeping production | Entry, review, reconciliation, ledgers | 25 % | 8,000 - 22,000 |
| Client portal | Document upload, approval, signature | 20 % | 6,000 - 18,000 |
| DMS | Filing, OCR, search | 15 % | 4,500 - 13,500 |
| Billing & time | Engagement letters, time tracking | 15 % | 4,500 - 13,500 |
| Dashboards | Engagement profitability, firm KPIs | 10 % | 3,000 - 9,000 |
| Integrations (bank, ledgers, HMRC) | APIs, automated imports | 15 % | 4,500 - 13,500 |
The client portal is the biggest differentiator: secure document upload, automated reminders and e-signature cut e-mail back-and-forth in half.
Custom vs market suite: the real math
The decision hinges on headcount and amortisation horizon. The table compares total cost over 5 years (2026 estimate).
| Criterion | Market suite | Custom (Kolonell) |
|---|---|---|
| Entry cost | 0 EUR | 30,000 - 90,000 EUR |
| Subscription | 40-120 EUR/staff/month | Maintenance 700-1,800 EUR/month |
| 15 staff / 5 years | 54,000 - 108,000 EUR | 72,000 - 198,000 EUR |
| Customisation | Limited to options | Total |
| Custom client portal | Rare / standard | Yes, differentiating |
| Code ownership | No | Yes |
| Development day rate | N/A | 500-700 EUR |
Below 15 staff, the suite almost always wins. Beyond that, or if the firm sells a premium service around the portal, custom amortises in 3 to 4 years and becomes an asset.
Mini case study
Claire, managing partner of a 22-staff accounting firm in central London. She pays 90 EUR per staff member per month today, i.e. 23,760 EUR/year. Custom software at 68,000 EUR with maintenance at 1,200 EUR/month (14,400 EUR/year) costs 82,400 EUR in year one, then 14,400 EUR/year. Over 5 years: 125,600 EUR custom vs 118,800 EUR suite. The 6,800 EUR premium is absorbed by the portal gain, which lets her bill a "digital monitoring" package of 25 EUR/month to 120 clients, i.e. 36,000 EUR/year in extra revenue. Custom becomes profitable from year 2.
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FAQ
How long to build accounting-firm software?
Expect 12 to 24 weeks in 2026 depending on scope. An MVP (production + portal) ships in 12-14 weeks; advanced DMS and bank integrations add 6 to 10 weeks.
Can we integrate ledgers and bank APIs?
Yes. Ledger imports, statement retrieval via open banking APIs and tax authority connectors are standard. They account for around 15 % of budget, i.e. 4,500 to 13,500 EUR.
Is custom GDPR compliant?
Yes, with EU hosting, encryption and access logging. It is even an argument against cloud suites whose data location you do not control.
What does maintenance cost after delivery?
Maintenance runs 700 to 1,800 EUR/month depending on the SLA. It covers fixes, legal updates (chart of accounts, VAT) and support.
When should we switch from a suite to custom?
Usually beyond 15 staff, or when a specialty (agriculture, real estate, associations) imposes workflows the suite cannot cover.
Let's scope your project. Tell us about your firm (headcount, priority modules, indicative budget 30-90,000 EUR, target timeline) and we'll 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.
