Valley Green Nurseries Ltd said it needed the new glasshouse off Paynes Lane, Nazeing, to grow more tomatoes and peppers.
The Lee Valley Regional Park Authority objected, but permission was granted by Epping Forest District Council in August 2014.
The park authority pointed out that the glasshouse will stand in the midst of the green belt, within the regional park and less than a mile from the Lee Valley Special Protection Area.
It fought to overturn the decision at the High Court last year, without success, and that ruling has now been confirmed by Appeal Court judges in London.
The park authority argued the council had misapplied or misunderstood local and national planning policies.
The glasshouse would damage the "openness" of the green belt and breach European rules for the protection of wild flora and fauna habitats, it was argued.
But Lord Justice Lindblom pointed out that new buildings for agriculture and forestry are not viewed as "inappropriate" in the green belt.
There was nothing "deficient or misleading" about the advice given to the council by one of its planning officers, he added.
The council had complied with its duties under European habitat rules and was entitled to find that the development would be sustainable.
Lords Justice Treacy and Underhill agreed that the park authority's appeal should be dismissed.
R on the Application of Lee Valley Regional Park Authority v Epping Forest District Council. Case Number: C1/2015/1430