The verdict in three sentences
Under 15 staff, a vertical licence is still the best value/risk ratio: fast deployment, compliance kept current. Between 15 and 40 staff with specific processes, an integrated Odoo or a custom layer become profitable over 3 years. Beyond that, if your productivity depends on a proprietary workflow, a custom build (EUR 35,000-80,000) pays back in hours saved.
The three options, real 2026 prices
The London accounting-firm market splits into three solution families. The figures below are 2026 order-of-magnitude estimates, excluding volume discounts.
| Option | Entry cost | Recurring cost | Rollout time |
|---|---|---|---|
| Vertical licence (Cegid, Xero, IRIS) | EUR 0-5,000 setup | EUR 30-120/user/month | 2-6 weeks |
| Odoo + integration | EUR 8,000-30,000 | EUR 24-45/user/month | 6-12 weeks |
| Custom development | EUR 35,000-80,000 | Maintenance EUR 800-2,500/month | 12-20 weeks |
| Custom layer on a vertical | EUR 12,000-25,000 | EUR 400-1,200/month | 6-10 weeks |
The vertical licence carries compliance (statutory filings, audit files) with no effort, but locks you into its ergonomics. Custom flips the equation: high upfront cost, but a tool that fits exactly how you produce.
3-year TCO for a 20-person firm
The real trade-off is total cost of ownership, not the sticker price. For 20 users over 36 months:
| Line item | Vertical licence | Integrated Odoo | Custom |
|---|---|---|---|
| Setup / development | EUR 3,000 | EUR 22,000 | EUR 60,000 |
| Subscription / licences 3 yrs | EUR 43,200 | EUR 25,200 | EUR 0 |
| Maintenance 3 yrs | included | EUR 18,000 | EUR 54,000 |
| Training | EUR 4,000 | EUR 6,000 | EUR 8,000 |
| 3-year total | EUR 50,200 | EUR 71,200 | EUR 122,000 |
On paper the licence wins. But this table ignores the value of hours saved: a custom tool that removes 30 minutes of re-keying per person per day represents, across 20 people at EUR 55/h loaded, roughly EUR 150,000 of freed time per year. That is where custom tips over.
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Mini case study
Claire, partner in a 22-person accounting firm in central London. Her teams lose on average 40 minutes/day re-keying between the production tool and the document store. At EUR 55/h loaded, that is 22 x 0.66 h x EUR 55 x 220 days = EUR 175,000/year of unproductive time. A custom tool at EUR 60,000 plus EUR 18,000/year maintenance that removes 70 % of that loss frees EUR 122,000/year of capacity. The project is repaid in under 8 months, then yields tens of thousands of euros of net margin per year.
FAQ
Is Odoo really suitable for a UK accounting firm? Yes for management, invoicing and client portals, but statutory compliance stays weak: budget EUR 8,000-30,000 of integration and often a vertical in parallel for regulated production.
How much does custom software maintenance cost? Plan for EUR 800-2,500/month depending on complexity, roughly 15-20 % of the development cost per year for enhancements, security and support.
How fast is a 20-person firm live? Vertical licence: 2-6 weeks. Odoo integration: 6-12 weeks. Full custom: 12-20 weeks, with interim releases from week 6.
At what headcount does custom become worthwhile? Generally above 15-20 staff with specific processes: the value of hours saved (often EUR 100,000+/year) then clearly exceeds the upfront premium.
Can we keep our vertical and add custom on top? Absolutely. A custom layer (client portal, automations, dashboards) costs EUR 12,000-25,000 and plugs into your vertical via API, without replacing everything.
Let's scope your project. Tell us your headcount, current tool and your two most costly time drains, and we cost the most profitable option (layer, integration or full custom). 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.
