The verdict in three sentences
Custom software for a London accounting firm costs between GBP 35,000 and 90,000 in 2026, versus GBP 20 to 55 per staff member per month for a standard suite. The real gain is time: 8 to 12 hours per week per staff member freed from document chasing and data entry. Beyond 12 staff, the bespoke investment becomes cheaper than a SaaS subscription over a 3-year horizon.
Bespoke or off-the-shelf suite: the real math
Market suites (Xero, Sage, IRIS, BrightManager) bill on recurring subscriptions. Bespoke is a one-off investment, amortised, with 15-20% annual maintenance. Here are the 2026 orders of magnitude.
| Solution | Entry cost | Recurring (15-person firm) | Customisation |
|---|---|---|---|
| Standard SaaS suite | GBP 0 | GBP 4,800 - 9,600/yr | Low (config) |
| Premium suite + modules | GBP 2,600 setup | GBP 10,800 - 16,000/yr | Medium |
| Bespoke Starter | GBP 35,000 - 48,000 | GBP 6,000 - 8,000/yr | Full |
| Bespoke Growth | GBP 48,000 - 66,000 | GBP 8,000 - 11,000/yr | Full + API |
| Bespoke Premium | GBP 66,000 - 90,000 | GBP 11,000 - 16,000/yr | Full + AI collection |
Over 3 years, a 15-person firm pays GBP 32,000 to 48,000 for a premium suite while owning nothing. A bespoke Growth build at GBP 58,000 + GBP 27,000 maintenance reaches GBP 85,000 but absorbs growth with no per-seat surcharge.
What gets built and what it saves
A typical firm tool covers four blocks. Each has a measurable impact on busy-season productivity.
| Module | Key function | Estimated gain |
|---|---|---|
| Document collection | Auto reminders, invoice OCR | 3 - 4 h/wk/staff |
| Case workflow | Steps, deadlines, assignment | 2 - 3 h/wk/staff |
| Smart DMS | Auto-filing, full-text search | 1 - 2 h/wk/staff |
| Client portal | Upload, signature, queries | 2 - 3 h/wk/staff |
| Profitability reporting | Time spent vs fees | 5 - 8% margin |
For a 15-person firm at GBP 40/hour loaded cost, 10 hours saved per week per person represents about GBP 6,000/week of recovered capacity, or over GBP 270,000 across a working year.
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Mini case study
Claire runs a 14-person accounting firm in the City of London. In 2025, every portfolio increase forced a hire. She invests GBP 55,000 in a bespoke tool (collection + workflow + portal) delivered in 4 months. Result over 12 months: +20% of cases absorbed with no new hire, an estimated loaded-salary saving of GBP 42,000/year, and bilateral production time down 22%. The tool pays back in about 20 months counting only the avoided hire.
FAQ
How long to build such a tool? Expect 3 to 6 months depending on scope: 3 months for a collection + portal block, 5 to 6 months for the full set with DMS and reporting. A usable MVP can ship in 8 to 10 weeks.
Can we keep our existing accounting production software? Yes. The bespoke tool sits on top via API or file imports; it orchestrates collection and workflow without replacing your entry engine if that suits you. This is often 30% cheaper than a full replacement.
What maintenance budget should we plan? Between 15 and 20% of build cost per year, i.e. GBP 8,000 to 11,000 for a GBP 58,000 tool. This covers regulatory changes, security and fixes.
Is it GDPR-compliant and confidential? Yes, with UK/EU hosting, document encryption and fine-grained per-case permissions. These requirements are designed in, not bolted on.
From how many staff is bespoke justified? Generally beyond 10 to 12 staff, when cumulative per-seat subscription cost exceeds owning a tool. Below that, a good SaaS suite stays cheaper.
Let's scope your project. Tell us your headcount, current software and busy-season pain points, and we'll scope a build with an indicative budget of GBP 35,000 to 90,000. 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.
