A rent repayment order is one of the most powerful tools available to tenants in England. If your landlord has committed certain housing offences, you can apply to the First-tier Tribunal to recover rent you've paid while the offence was being committed.
The offences that can trigger an RRO include operating an unlicensed HMO, managing or controlling an unlicensed property in a selective licensing area, illegal eviction or harassment, failing to comply with an improvement notice, failing to comply with a prohibition order, breaching a banning order, and (under the Renters' Rights Act) knowingly or recklessly misusing a possession ground.
The Renters' Rights Act increased the maximum claim period from 12 months to 24 months' rent and extended the time limit for making an application to 24 months from the date of the offence. The tribunal doesn't require a criminal conviction to make an RRO, but it must be satisfied beyond reasonable doubt that the offence was committed.
The application costs £100, with a further £200 if a hearing is required and, if successful, you can recover these fees. You don't need a solicitor, although seeking advice from Shelter or Citizens Advice before applying can help strengthen your case.