Kerr was speaking at an HLF get-together at Fulham Palace, which in October won a £270,000 lottery grant, with £3,648,000 earmarked for the main project at its grounds and neighbouring Bishops Park.
The project will restore a moat, turn a gothic lodge into a gardener's cottage and create a beach area next to the River Thames. Kerr said the HLF was targeting the London boroughs of Ealing, Sutton, Enfield, Havering and Bexley to apply for grants.
He explained: "Unless historic places get new audiences there won't be new volunteers to help look after them."
Kerr said a new "simpler" process to apply for grants was helping applicants, with online and mentoring support and decisions within three months.
He added that the Royal Parks' Bushy Park revamp - to be completed after a £4.5m lottery restoration in 2009 - is "going to be one of the great gardens in the world".