How are costs calculated on a partial indemnity versus substantial indemnity basis?

Partial indemnity costs recover only a portion of a party's actual reasonable legal fees, and represent the default, more modest scale a court applies in an ordinary costs award. Substantial indemnity costs recover a significantly higher proportion of actual fees, and are generally reserved for situations where a party's conduct in the litigation was particularly unreasonable, where they rejected a clearly favourable settlement offer and did worse at trial, or otherwise crossed a line beyond ordinary hard-fought advocacy.

In both cases, the court doesn't simply add up whatever a lawyer billed; it looks at what's reasonable and proportionate for the case, considering its complexity and the amount of work genuinely required, and applies the appropriate scale to that reasonable amount. Which scale applies, and how a court exercises its discretion within that scale, depends heavily on the specific facts, including the parties' conduct and any offers to settle exchanged during the case. Anyone wanting a realistic estimate of potential costs exposure should discuss their specific situation with a family lawyer rather than assuming a fixed formula applies.