Dormice are found in Africa, Asia and Europe and are known for their long hibernation periods.
Volunteers had carried out work at reserves in the north of the county over the past few years to ensure the habitat was suitable for the rodents.
In the spring of this year, the trust, along with Thames Chase Forest Centre, near the M25, planted the nest boxes in the hope of seeing numbers rise.
“By October, we had counted 15 individual dormice, including juveniles,” a spokesperson for the trust said.
“This means they’re not just there, they’re breeding and thriving.”