"Temperate House" in London's Kew Gardens is large enough to house three jumbo jets, and was home to around 1,000 species of plants from around the world before it was shut in 2013 after falling into a state of disrepair.
"There was rust everywhere, all the paint was falling off, and look now, it's all brand spanking new," project manager Andrew Williams told AFP, as a fleet of diggers and teams of workers put the finishing touches to the £41 million ($57 million, 46 million euros) renovation project.
The wrought iron and glass structure was designed by esteemed Victorian architect Decimus Burton in 1860 and opened in 1863.
The facelift required the removal of 69,000 individual elements to be cleaned, repaired or replaced and the restoration of 15,000 panes of glass.
Enough paint to cover four football pitches was used to spruce up the huge iron columns, and Kew expects hundreds of thousands of visitors to pass through its doors annually after its May reopening.
"A building like this deserves it," said Williams.
"I don't think you'd build a building like this now," he added.
"I had a love-hate relationship. You end up loving it at the beginning (of the project), then hating it in the middle and loving it at the end. It's hard work.
"Everybody who has worked in here is really proud and now you see the plants going in, it's a fantastic space."
With weeks to go before the grand reopening, horticulturalists are hard at work rehousing the plants, many of which were transferred to on-site nurseries during the renovation work.