The verdict in three sentences
For a London firm under 10 staff, a vertical licence (IRIS, Xero Practice, BTCSoftware) at GBP 30-80 per user per month remains the rational choice. Beyond 15 staff, or if you want a differentiating client portal and native 2026 e-invoicing integration, custom software at GBP 40,000-120,000 wins over 3-4 years. The real question is not purchase price but total cost over four years, subscriptions and lost hours included.
Custom vs vertical licence: the 2026 cost comparison
Vertical vendors cover 80 % of standard needs. Custom is justified when your specific 20 % (review workflow, per-engagement billing, structured document management) weighs heavily on productivity.
| Criterion | Custom build | Vertical licence (IRIS/Xero) |
|---|---|---|
| Upfront cost | GBP 40,000-120,000 | GBP 0-3,000 (setup) |
| Recurring cost | Hosting + maintenance 12-18 %/yr | GBP 30-80/user/month |
| Time to deploy | 4-8 months | 3-8 weeks |
| Code ownership | You | Vendor |
| Workflow customisation | Unlimited | Config-limited |
| 2026 e-invoicing | Custom, native | Included (vendor roadmap) |
| Cost at 20 users over 4 yrs | GBP 80,000-160,000 | GBP 96,000-192,000 |
For a 20-staff firm, a GBP 60/user/month licence costs GBP 57,600 per year, or GBP 230,400 over four years. A custom build at GBP 90,000 plus 15 % annual maintenance amounts to around GBP 144,000 over the same period.
Productivity gains and e-invoicing: what changes in 2026
UK e-invoicing and Making Tax Digital push standardised flows. Software tailored to your firm automates document collection, OCR pre-bookkeeping and client chasing.
| Task | Before tool | After custom tool | Monthly saving |
|---|---|---|---|
| Client document collection | 6 h/staff | 2 h/staff | 4 h |
| Entry and pre-bookkeeping | 8 h/staff | 3 h/staff | 5 h |
| Per-engagement billing | 4 h | 1 h | 3 h |
| Chasing and follow-up | 3 h | 0.5 h | 2.5 h |
| Partner reporting | 3 h | 0.5 h | 2.5 h |
| Total | 24 h | 7 h | 17 h/staff |
Saving 15-25 hours per staff member per month, valued at a GBP 45 loaded hourly cost, recovers GBP 675-1,125 per person each month.
Mini case study
Mark, partner in an 18-staff accounting firm in the City of London, weighed renewing his vertical licence (GBP 61,000/year for 18 seats) against custom development. He chose dedicated software at GBP 88,000, delivered in 6 months, with 14 % annual maintenance (GBP 12,320). Over 4 years: GBP 88,000 + 49,280 = GBP 137,280, versus GBP 244,000 in licence fees. Gross saving: GBP 106,720. By recapturing 17 h/month per staff into billable advisory work, the firm adds roughly GBP 165,000 in extra annual margin from year two.
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FAQ
How much does custom accounting practice software cost in London in 2026?
Expect GBP 40,000-120,000 depending on scope (document management, client portal, per-engagement billing, e-invoicing). A targeted MVP sits around GBP 45,000-60,000.
How long does it take to build and deploy?
Between 4 and 8 months: 4-6 weeks of scoping, 3-5 months of development, then data migration and training. Phasing enables a first go-live by month three.
Is custom software compliant with 2026 e-invoicing?
Yes, provided you connect an approved platform and the correct structured format. It is an advantage: you embed the reform in your existing flows rather than waiting on a vendor roadmap.
What maintenance budget should I plan after delivery?
Between 12 and 18 % of development cost per year, i.e. GBP 5,000-21,000. This covers fixes, regulatory updates and secure hosting.
At what headcount does custom become worthwhile?
Usually beyond 15 staff, or earlier if your specific processes generate heavy manual entry. Below 10 seats, the vertical licence stays cheaper.
Let's scope your project. Tell us your firm size, priority modules (document management, client portal, e-invoicing) and target budget, and we will size the optimal 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.
