Development funding of £151,000 was awarded to the charity Historic Royal Palaces to help fully develop plans to open up the castle and its gardens, ready to welcome 200,000 visitors each year by 2019.
The project can now proceed to the next stage of the application process to secure the full £5m grant.
Hillsborough Castle is part of a 40ha estate made up of historic buildings, gardens and parklands in County Down. The ambitious £16m project will see the site transformed and made more accessible with new visitor facilities including car parking, a learning centre and retail space.
The walled garden will be restored and a section of the grounds known as the Lost Garden, which had become overgrown and difficult to access, will also be returned to its former glory. Inside the castle itself, visitors will be able to explore the state rooms, which will be newly presented, opened up and interpreted to provide an insight into their use as both a royal residence and as a venue for entertaining and political discussion.
The project has a focus on creating skills development opportunities for the local community. The gardens will become a hub for training activities, with an emphasis on horticulture, traditional craft and heritage skills, food and the environment, teamwork and confidence building. More volunteers will also be recruited and trained in a variety of roles including tour-guiding, assisting with visitor information and working alongside the gardening, conservator and events teams. In addition, the new Clore Learning Centre will host activities for schools, families, youth and community groups, as well as talks and workshops for adults.
Head of HLF Northern Ireland Paul Mullan said: "Thanks to National Lottery players these exciting plans to open up Hillsborough Castle and its unique heritage can progress to the next stage of development. We are delighted to support this project which will deliver real benefits for people through improved access, facilities and interpretation and create exciting opportunities for skills development, training and volunteering."
Michael Day, chief executive of Historic Royal Palaces, said: "We are thrilled that HLF is supporting our plans to transform Hillsborough Castle into an engaging and inspirational visitor destination and place for learning. Hillsborough Castle holds a unique place in the history of Northern Ireland and we are looking forward to sharing its rich and varied stories with the widest possible audience."