Which organization is NOT mentioned as involved in leasehold management?

The Royal Institute of British Architects is the correct choice because this organization primarily focuses on the architecture profession and related practices. Its main objectives include promoting architecture and providing support to architects but does not specifically engage in leasehold management or issues surrounding leasehold properties.

In contrast, the other organizations mentioned—National Housing Federation, Local Government Association, and Chartered Institute of Housing—are directly involved in housing policies and management practices, including leasehold management. They focus on aspects such as affordable housing, local government housing strategies, and professional guidance on housing management. This distinction highlights the specific role of each organization in the broader context of housing and leasehold issues, making the Royal Institute of British Architects the least relevant entity with respect to leasehold management.