How to Build a 100-Day Integration Plan

Start from the deal thesis, not a template dump

Most 100-day plans fail before they are written, because they start as a list of everything that could be done rather than the few things the deal was priced on. Before any milestone goes in a spreadsheet, write down the three or four sources of value the acquisition case depends on: a cost synergy number, a cross-sell motion, a capability, a market position. The plan exists to protect and deliver those. Everything else is hygiene, and hygiene gets a lighter touch.

A useful test: if a milestone slipped four weeks, would anyone outside its workstream care? If not, it belongs in a workstream tracker, not the integration plan.

The structure: phases across, workstreams down

The plan that works in practice is a grid. Across the top, five phases: pre-close, Day 1, Day 30, Day 60 and Day 100. Down the side, the workstreams, typically eight to ten of them: leadership and governance, communications, HR and people, finance, IT and systems, legal and regulatory, commercial and sales, operations, customers, and synergy capture.

Each cell holds a small number of milestones, not tasks. A milestone is something a steering committee would recognise as done or not done: “combined org design approved”, not “hold org design workshops”. Task-level detail lives with the workstream lead. When the integration plan tries to hold both, it becomes a four-hundred-line Gantt chart that nobody owns and nobody reads, which is the most common way these plans die.

What a milestone row actually needs

Five columns do almost all the work:

An owner who is a name, not a department. “Finance” cannot be chased. A named person can.
A date, even if it is wrong. A wrong date generates a conversation; “TBC” generates nothing.
A dependency, if one exists. Cross-workstream dependencies are where integrations break, and the plan is the only place they are all visible at once.
A definition of done. One sentence. It prevents the Day 60 argument about whether something counts.
A RAG status with rules. Red should mean “a decision or intervention is needed above workstream level”, not “behind”. If red just means bad, everything drifts to amber and the status loses meaning. Boards can smell RAG inflation.

Pre-close: plan, do not act

Between signing and completion you are still two separate companies, and in many jurisdictions there are hard limits on information sharing and coordination before close. What you can almost always do is plan: agree the integration charter, name the workstream leads, draft the RACI, write the Day 1 communications, and build this plan. Deals with regulatory clearance timelines get months of this window; use it, because the teams that arrive at close with a signed-off plan are the ones whose first thirty days look calm.

Day 1 to Day 30: stabilise

The first month is about proving that the ordinary things still work: people are paid, systems are accessible, customers hear a consistent story, and the top retention targets have had a personal conversation about their future. Synergy work in this window is mostly preparation, not delivery. The exception is anything with a long lead time, such as contract notice periods, which needs to start immediately even though it will not land for months.

Day 30 to Day 100: integrate and bank value

From the Day 30 review onwards, the plan's centre of gravity shifts to the deal thesis. Each synergy initiative needs an owner, a number, and a date on which that number appears in the P&L, tracked in the same rhythm as the rest of the plan rather than in a separate finance file. Keeping the synergy tracker inside the integration governance is not an administrative preference: when finance tracks value in one file and the integration team tracks progress in another, the two diverge by Day 60 and the steering committee stops trusting both.

This window is also where the organisational decisions land: the combined structure, the systems roadmap, which offices and brands persist. Deferring these past Day 100 is occasionally right and usually rationalisation.

Uncertainty is more corrosive to retention than most bad news.

The cadence that keeps the plan alive

A plan updated monthly is a historical document. The rhythm that works: workstream leads update their rows weekly, an integration meeting reviews movement and dependencies weekly, and the steering committee meets monthly against a one-page dashboard fed from the plan, with phase-gate reviews at Day 30, 60 and 100. The point of the gates is not ceremony; it is a scheduled moment where the plan is allowed to change. Plans frozen at kick-off are the second most common way these plans die.

What Day 100 should look like

Not finished; integrations are not finished at Day 100. The realistic standard: the deal-critical milestones delivered or consciously re-planned, synergies tracking against the model with finance sign-off, remaining risks owned in business-as-usual registers rather than an integration file, and a lessons-learned review written while memories are honest. Then the plan is retired, deliberately, rather than left to fade.

The templates that implement this

The toolkit's 100-Day Integration Plan, Day 1 Readiness Checklist, Risk Log and Board Dashboard follow the structure described here, pre-populated with a worked example. One-time purchase from £199.

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