East Anglia's Children's Hospices (Each) wants to replace its aging facility in Quidenham, Norfolk, to help children with terminal illnesses.
The duchess, who is the charity's patron, will attend the launch at the Norfolk Showground on 25 November.
While there, she will meet families and speak to supporters of the hospice.
The charity runs services in Norfolk, Suffolk, Cambridge and Essex.
The current Quidenham hospice is one of the oldest in the UK and is extremely restricted in its facilities and development, the charity said.
It is hoped the new building, in Framlingham Earl, would include a hydrotherapy pool and improved accommodation and play areas.
The Duchess of Cambridge is to attend the launch of a £10m fundraising appeal to build a hospice for young people in Norwich.