When can you start charging interest on unpaid costs?

• Non-contentious: you can charge interest from one month after delivery of the bill of costs (provided the client has been given written notice of the right to request a remuneration certificate, and the right to seek assessment of the bill by the court). See Article 14 Solicitors (Non-Contentious Business) Remuneration Order 1994.
• Contentious:

  there is no such statutory right to charge interest on outstanding costs. But, you can charge interest if that right was expressly reserved in the retainer agreement (or if the client later agreed to pay interest, having given consideration, or if you have sued the client and claimed interest in the usual way).