Les Caches farm and barn are one of four sites that have been added to the protected building register.
President Tony Spruce said he had never even thought to check whether the historic farm site, which is close to Le Gouffre in the Forest, was on the register.
'I'm amazed at that,' he said.
The 'new' house is closer to the road and dates from the 1800s, while the 'old' thatched house sits further back in the site.
Parts of it date from the 1400s.
The planners said the site was a good example of the evolution of farm buildings.
The National Trust has been doing up renovating the old building for more than a decade and hopes to open it up this summer.
Mr Spruce said they just needed to finish some work, but that would not be affected by the listing.
Les Beaucamps de Bas – a rare, formal country house off Les Vieux Beaucamps – is another property that has been added to the list.
