The verdict in three sentences
Custom software for a London accounting firm costs, in 2026, between 45,000 and 110,000 EUR net, delivered in 5 to 8 months, and does not replace your production suite: it targets the client relationship, document collection and the review workflow. The return on investment comes from a 30 to 50 % saving in data-entry time thanks to automated invoice collection. Budget 10 to 15 %/year in maintenance to keep it compliant and evolving.
What custom software adds when you already run a production suite
Market suites remain essential for statutory production, tax filings and payroll. Custom software fills what they handle poorly: client experience, document centralisation and orchestration of review tasks. That is where a firm's margin is won or lost.
| Function | Production SaaS only | + Custom module 2026 |
|---|---|---|
| Client document collection | Email / manual drive | Portal + automatic OCR |
| Data-entry time | 100 % baseline | -30 to -50 % |
| Client request tracking | Scattered inbox | Structured thread per file |
| Review workflow | Excel cells | Automated status & alerts |
| Partner reporting | Manual export | Real-time dashboard |
| Year-end close time | Baseline | -1 to -2 weeks |
What it really costs in 2026
The budget depends mostly on functional scope and the number of integrations with your existing tools. Here are the 2026 orders of magnitude for the London market.
| Scope | Net budget 2026 | Timeline | Maint./yr |
|---|---|---|---|
| Client portal + document collection (OCR) | 45,000 - 60,000 EUR | 5 months | 10 % |
| + Review workflow + alerts | 60,000 - 80,000 EUR | 6-7 months | 12 % |
| + Engagement billing + partner reporting | 80,000 - 110,000 EUR | 7-8 months | 15 % |
| Production-suite integration (API/connector) | 6,000 - 15,000 EUR | included | - |
| E-signature for engagement letters | 4,000 - 9,000 EUR | included | - |
Mini case study
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Mark, partner at a 22-person accounting firm in London, handles 480 client files. His team spends on average 6 hours per file per month chasing and keying in documents. An automated collection module cuts that by 40 %, i.e. 2.4 hours saved per file per month: 480 x 2.4 = 1,152 hours/month. Valuing even half of that at 35 EUR/hour loaded cost yields roughly 20,000 EUR/month of freed capacity. Against a 70,000 EUR investment plus 8,400 EUR first-year maintenance, breakeven is reached in under 5 months of effective rollout.
FAQ
Does custom software replace my production suite? No. Those suites handle statutory production (filings, VAT, payroll). Custom software connects to them via API to orchestrate client relations and review, where you lose 30 to 50 % of time today.
How long until it is operational? Expect 5 to 8 months depending on scope. A first release (portal + collection) can ship in 5 months for quick impact on the tax season.
Is the data GDPR-compliant? Yes: EU/UK hosting, encryption, access logging and fine-grained permissions. This is a non-negotiable prerequisite for a firm.
What maintenance budget should I plan? Annual maintenance of 10 to 15 % of the development cost covers fixes, regulatory updates and support, i.e. roughly 7,000 to 16,500 EUR/year on a 70,000 to 110,000 EUR project.
Can we start small then extend? Yes. We recommend a collection+portal core, then progressively adding the review workflow and partner reporting once gains are measured.
Let's scope your project. Share your headcount, current production suite and file volume for a targeted estimate. 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.
