A mandatory legal document in the UK confirming that a property's gas appliances and fittings have been annually checked and deemed safe by a Gas Safe registered engineer.Landlords are legally required to obtain this certificate (often called "CP12") for their rental properties every 12 months, provide a copy to tenants within 28 days (or before move-in for new tenants), and keep records for two years.Failure to comply is a serious offence and can invalidate Section 21 eviction notices.