The Arithmetic Most Firms Skip
Self Assessment feels like recurring revenue. Predictable fees, stable demand, work that returns each January. But few firms model what a return actually costs to deliver.
Not the charge-out rate. The fully loaded cost.
When salary, utilisation (typically 60-70% for preparers) and overhead allocation (30-50% of fee income) are properly accounted for, the effective cost of a productive hour in many UK firms approaches £100. That figure is rarely written down. It matters.
Three to Six Hours, Depending
Based on discussions with UK firms, a typical Self Assessment return requires three to six hours of internal time. That includes data collection, document classification, preparation, review, client approvals and filing. Not just the software entry.
At three hours and a £100 fully loaded cost, that's £300 per return. At six hours, £600. Before write-offs, scope creep or untracked partner input.
The Margin Question
To achieve a 33% gross margin on a £300 cost base, fees need to sit around £450. For a £600 cost base, closer to £900.
Many firms do not charge £900 for a complex return. Those that do are often the ones who have done this arithmetic.
Average UK accountant fees for Self Assessment range from £150 (basic) to £500 (complex), with full-service annual packages running £800-£2,500 according to recent market data. Fixed pricing risks under-servicing complex cases or over-charging simple ones.
The MTD Complication
From April 2026, Making Tax Digital mandates digital records and quarterly updates for sole traders and landlords over £50,000 turnover, dropping to £30,000 then £10,000 in phases. That adds compliance overhead to an already tight margin structure.
HMRC's late filing penalties start at £100 (one day overdue), escalating to £10 daily after three months. Clients who miss deadlines create unplanned workflow spikes. Firms absorb that cost.
The work returns each year. The question is whether the economics do.