The detached property near Fort Doyle has been sold to a Thai buyer for £1,010,000.
The site was formerly owned by the States, which sold it to a private company for £535,000 in 2018.
The property was put back on the market in April for more than £1.1m., with planning permission having been granted to develop the site.
The Alderney Bird Observatory had expressed interest in the site, saying it would be the perfect place for watching birds in Guernsey.
‘[Platte Fougere] would have been an ideal site, but it’s not an ideal world,’ said Alderney Bird Observatory warden John Horton.
‘It would have ticked a lot of boxes, but there are other good sites to consider that are better for other reasons.’
Mr Horton said the group had already began talks about purchasing another site for a Guernsey Bird Observatory, but could not say more at this time.
‘We have had some really productive meetings in Guernsey recently,’ he said.
‘It’s all looking very positive.’