The landscape and environmental firm will spend 10 months restoring a 2ha section of Hotham Park in Bognor Regis.
The area is cut off from the rest of the park, dotted with derelict buildings and swathed in overgrown brambles, grass and weeds.
Landscape designers will create a wildlife conservation zone with a pond, wildflower meadows and native trees as well as an open grass area for events.
When complete, around August this year, it will be the first time for nearly 60 years that the area has not been divided from the rest of the park by walls or fences.
Chris Blandford Associates drew up the master-plan for the client, Arun District Council, which was supported by the Heritage Lottery Fund in 2006.
UPM Tilhill recently finished refurbishing Kensington Memorial Park's south-west corner. The 25-week job included new landscape and railings and an interactive water area with 22 play features.