The verdict in three sentences
A custom line-of-business software for a London accounting firm costs, in 2026, between 18,000 and 65,000 EUR, with 70 % of the spread explained by user count and billing/payroll integrations. The real trap is not the development price but the annual maintenance (15 to 20 % of the project), too often left out of budgets. An 8-person firm that automates its accounting production recoups the tool in 11 to 16 months.
The cost lines of a 2026 quote
A serious quote is never a single number. It breaks down into lots you should be able to challenge line by line. Here is the standard structure we apply at Kolonell for a London firm.
| Cost line | Range EUR | What drives it |
|---|---|---|
| Scoping & requirements | 2,000 - 4,500 | Number of business workshops |
| UX/UI design | 3,000 - 7,000 | Screens, dashboards |
| Core development | 9,000 - 30,000 | Modules, business rules |
| Billing/payroll integration | 2,500 - 9,000 | Xero, Sage, payroll APIs |
| Data migration | 1,500 - 6,000 | Volume, legacy quality |
| Staff training | 1,200 - 3,500 | Number of users |
| Acceptance & go-live | 1,500 - 4,000 | Workflow complexity |
On top of that comes maintenance, often quoted separately. Budget 15 to 20 % of development cost per year, i.e. 3,000 to 9,000 EUR for an average project.
The impact of user count and integrations
The number of staff accessing the tool drives training, infrastructure licensing and permissions management. Integrations turn a simple internal tool into the true backbone of the firm.
| Firm profile | Users | Typical integrations | Project budget EUR |
|---|---|---|---|
| Micro-firm | 1 - 3 | Billing only | 18,000 - 26,000 |
| SME firm | 4 - 10 | Billing + payroll + DMS | 30,000 - 45,000 |
| Structured firm | 11 - 25 | + e-signature, client portal | 48,000 - 65,000 |
| Multi-site group | 25+ | + consolidation, SSO | 65,000+ |
A payroll integration alone represents 2,500 to 5,000 EUR depending on depth (read-only vs two-way sync). A secure client portal for document upload adds 4,000 to 8,000 EUR but cuts email back-and-forth by 30 to 40 %.
Mini case study
Sophie, managing partner of an 8-person accounting firm in London, loses about 6 hours per week per staff member re-keying data between her billing tool, her production tracking spreadsheet and her DMS. Estimated loaded cost: 34 EUR/hour.
Annual loss: 8 staff x 6 h x 45 weeks x 34 EUR = 73,440 EUR of unproductive time. A line-of-business software at 38,000 EUR that cuts re-keying by 60 % saves roughly 44,000 EUR/year. With 6,000 EUR of annual maintenance, net ROI is reached in 13 months. From year two, the firm frees up 38,000 EUR of recovered value every year.
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FAQ
How much does accounting firm management software really cost in 2026?
For a 4-to-10-person firm, budget 30,000 to 45,000 EUR custom-built, excluding maintenance. An adapted off-the-shelf solution runs 8,000 - 15,000 EUR setup plus a 40 to 90 EUR/user/month subscription.
Should you prefer custom over an off-the-shelf solution?
Below 4 users, the vendor is often more cost-effective. Beyond 10 staff with specific processes, custom becomes competitive because it removes per-user subscriptions that reach 10,800 EUR/year for 10 people at 90 EUR/month.
What budget for annual maintenance?
Between 15 and 20 % of development cost, i.e. 3,000 to 9,000 EUR/year for an average project. This covers fixes, security updates and regulatory changes (e-invoicing 2026).
How long to build the tool?
An SME firm project spans 10 to 16 weeks from scoping to go-live, with a 2-to-3-week acceptance phase. Payroll integrations often extend the timeline by 3 to 4 weeks.
Does mandatory e-invoicing change the quote?
Yes. The 2026 obligation requires a connection to an approved platform, adding 2,000 to 4,500 EUR to the project but becoming a strong commercial argument with your clients.
Let's scope your project. Tell us about your firm (headcount, current tools, desired integrations) and your ballpark budget. 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.
