A controller's reserve is another name for budgetary slack, an amount built into a budget beyond what managers genuinely expect to need, so that overruns or revenue shortfalls can be absorbed without missing targets. The main entry covers the causes of slack, its costs and a worked example.
A reserve in the budget, not in the accounts
The word "reserve" here means an informal amount held back for safety, usually without being labelled as such. It is not a reserve in the financial-accounting sense. It does not appear on the balance sheet, is not an allowance or provision, and has nothing to do with retained earnings set aside by the board. It exists only in the planning numbers, typically as costs estimated a little high or sales estimated a little low across many budget lines.
Because the amount is hidden, it can be hard for senior management to find. Comparing budgets with actual results over several years, asking managers to explain each line from scratch through zero-based budgeting, and analysing each budget variance can all help to expose it.
Some caution in budgeting reflects the accounting principle of conservatism and is reasonable when the future is uncertain. A reserve becomes a problem when it is used to make performance look better than it is, or when it starves other parts of the business of resources. The same practice is also called budget slack or a controller's cushion.