Work will begin over the summer on the construction of a £15.7m remembrance centre at the National Memorial Arboretum in Staffordshire.
Arboretum chiefs said more than £10m of the amount needed for the development has been raised, meaning work could start.
The Duke of Cambridge launched the campaign when he visited the Alrewas site in April 2009.
Prince William also hosted a fundraising dinner which raised £4.2m.
'Overwhelmed by generosity'
The new development, which will include education facilities and changes to the reception, restaurants and shop, will allow the arboretum to cater for up to 500,000 visitors a year.
Grants and donations have been received from groups including Staffordshire County Council and the Heritage Lottery Fund.
The building work is expected to be completed by the end of 2016 and the arboretum will remain open during the development.
Sarah Montgomery, the arboretum's managing director, said: "We are thrilled to have reached this critical milestone in our bid to fund much-needed new facilities.
"We are now confident work can begin on site this year while we work hard to secure the remaining funds."
Maj Gen Patrick Cordingley, who chaired the appeal council, said: "We have been overwhelmed by the generosity of individuals and organisations alike in supporting our fundraising campaign."