What a Section 166 actually costs is rarely the number on the budget line. There are three ways to resource the same illustrative 20-month, multi-BU programme — and they produce three very different bills.

A Big 4 support layer is budgeted at roughly £3.9m. Programmes of this shape typically finish 30–45% over, so the realistic bill is nearer £5.4m. The overrun is not a surprise fee — it is the price of building the delivery machinery while the programme is already running.

An in-house build shows only about £0.8m of visible cost. But seconded staff are never truly free, the tooling is assembled under fire, and the overrun runs higher still — 35–50% is common once the re-planning cycles start.

The ACON model is a senior specialist running a pre-built system alongside your own team: budgeted from about £1.2m and engineered to close within 10% of it. The difference is not cheaper people — it is the absence of the overrun, because the foundation the programme pivots on arrives already built.

And the avoided overrun is the bigger number. At £500,000 a month, a "normal" 30% overrun is seven extra months — £3.5m more. That is the cost of the industry habit of building while delivering.

Read the full line-by-line arithmetic, with the three-bar cost comparison, at: https://www.aconconsulting.co.uk/cost-arithmetic

Registration to the ACON practitioner library is free — both illustrated white papers arrive by email the moment you register: https://dbe2-caitlin.systeme.io/dd4b9c68