The conservation charity needs the public to help raise up to £6m to safeguard the house and gardens of Seaton Delaval Hall in Northumberland.
The trust and local councillors believe the property and grounds could provide regeneration and sustainable development for the area. They have launched the trust's biggest consultation to ask the nation to decide how they would like to use the property and to take part in planning and fundraising.
Councillor Dave Stephens, leader of Blyth Valley Borough Council, said: "Seaton Delaval Hall can do for us what the Alnwick Garden has done for our neighbours to the north. It is a huge opportunity and we should all support it."
The National Trust has pledged £6m of its own funds but a further £6m must be raised by January to purchase the property.
- See www.nationaltrust.org.uk/seatondelaval.