Custom vs Off-the-Shelf Accounting Practice Software in London (2026)

The verdict in three sentences

For a standard practice under 15 staff, an off-the-shelf licence is hard to beat: fast go-live, built-in regulatory updates, no technical debt. Once the firm passes 20-25 staff or carries a strong specificity (own client portal, sector automations, complex re-billing), a custom build pays off over 5 years despite its entry ticket. The real question is not purchase price but total cost of ownership measured against productivity gained.

Licence vs custom: the 2026 numbers

Vendors now bill almost exclusively per seat per month. A custom build is paid once, then maintained. Order of magnitude for 2026, London market.

5-year TCO by practice size

The tipping point sits around 20 staff. Below it, the licence wins; above it, the custom build amortises. 2026 order-of-magnitude simulation (licence at 110 EUR/seat/month vs custom at 95 000 EUR + 15 % maintenance).

Mini case study

Claire, managing partner of a 32-staff accounting practice in central London. Her current licence costs 32 x 110 EUR = 3 520 EUR/month, i.e. 42 240 EUR/year and 211 200 EUR over 5 years. She commissions a custom core at 110 000 EUR + 15 % annual maintenance (16 500 EUR), including a document-management client portal and automation of recurring filings. 5-year TCO: 110 000 + (16 500 x 5) = 192 500 EUR. Gross saving of 18 700 EUR over 5 years, but above all 8 hours/week recovered on repetitive data entry, the equivalent of a part-time role valued at over 25 000 EUR/year.

FAQ

Can we keep the vendor tool and add custom around it? Yes, and it is often the best compromise: keep the vendor's tax engine and build only a client portal or automations via API. Typical 2026 budget: 25 000 - 45 000 EUR for the custom layer.

Does custom keep up with regulatory changes? Only if you budget for it: expect 5 000 to 12 000 EUR/year of dedicated evolutionary maintenance, versus automatic inclusion with a vendor.

Realistic timeline for a custom core? Between 4 and 8 months depending on scope. An MVP covering DMS, client portal and billing can ship in 4 months; advanced automations follow in phase 2.

From what headcount does custom become worthwhile? Around 20-25 staff over a 5-year horizon, or sooner if a business specificity justifies the build regardless of the licence maths.

Are you locked in with a licence? Partly: data reversibility is contractual but proprietary formats complicate migration. Custom makes you owner of the code and the database.

Let's scope your project. Tell us your headcount, current vendor and the 2-3 specificities costing you the most time, and we will price the custom core or complementary layer (indicative budget 25 000 - 140 000 EUR). Detailed quote within 48 h. WhatsApp +221 77 596 93 33.