Use 'tack on' when something is added surprisingly or as an afterthought.

Unexpected Addition

'Tack on' often refers to a small or minor addition, not a major change.

Slight Extension

In construction or design, 'tack on' implies a physical addition to something existing.

'Tack on' is casual; use more formal language in official or technical contexts.

'Tack on' can imply an unwelcome surprise in costs, such as hidden fees or charges.

Mention of 'tack on' can serve as a hint to be wary of additional expenses.

Use 'tack on' to inform or question about extra costs for transparency.