Accounting practice management software: build vs buy in London in 2026

The verdict in three sentences

Below 10 users, a SaaS at EUR 15-45/user/month is almost always the rational choice. Beyond 15-20 staff, or when your processes (engagement billing, engagement letters, payroll integration) fall outside the standard product, a custom build at EUR 40,000-90,000 becomes defensible over a 3-to-5-year horizon. The real trade-off is not the entry price but the TCO and the cost of time wasted working around an ill-fitting tool.

5-year TCO: SaaS vs custom build

For a 12-person practice, here is the 2026 order of magnitude (estimates, before negotiated discounts).

Over 5 years, SaaS is clearly cheaper in cash. A custom build is only justified if the productivity gain or strategic value (differentiation, reselling the module) exceeds the ~EUR 80,000 gap.

Break-even by number of users

SaaS charges per head: the larger the practice, the more competitive the fixed-cost custom build becomes.

The pure price crossover sits around 45-55 users. Below that, only an uncovered functional need (ERP integration, complex billing, specific compliance) justifies going custom.

Mini case study

Laurent, managing partner of a London accounting practice (12 staff, 380 clients), pays EUR 5,040/year in SaaS but loses time on engagement re-billing and reminders. A custom engagement-billing + client-portal module, quoted at EUR 58,000 (5-month lead time), automates these tasks. If it saves 2 hrs/staff/week valued at EUR 55, that is 12 x 2 x 46 x 55 = EUR 60,720/year of recovered time. The custom build pays for itself in just over a year despite the higher SaaS TCO.

FAQ

How much does custom accounting practice management software cost in London in 2026?

Expect EUR 40,000 to 90,000 depending on modules (engagements, billing, payroll, client portal), with a 4-to-7-month lead time. Annual maintenance runs about 12-15% of the development cost.

Isn't SaaS always cheaper?

In pure cash, yes up to ~45 users. But SaaS bills per head per month: beyond that, the custom build's fixed cost becomes more economical, not counting the value of automated bespoke processes.

Can existing payroll and billing be integrated?

Yes, via APIs or connectors. This is often the decisive argument for custom: centralising payroll, billing and engagement tracking in one tool, where SaaS forces costly re-keying.

How long until a first usable module?

An MVP covering engagement tracking and billing typically ships in 4 months. Advanced modules (client portal, dashboards, payroll integration) are added in 4-to-6-week iterations.

How do we avoid over-investing?

Start with the highest-ROI module (often engagement billing), measure the real gain, then decide on the rest. A good partner will tell you when SaaS remains the better choice.

Let's scope your project. Tell us your headcount, current tools (payroll, billing) and priority module, and we'll price the custom build and its break-even against your SaaS. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.