HMP Hollesley Bay was developed as a Borstal in 1938 on the site of a former colonial college with extensive farmlands whose original buildings, dated 1887, now house much of the administration centre. it became a young offender institution in 1988. In April 2002, Hollesley Bay open prison for adults mainly was formally separated from the adjacent HMYOI Warren Hill, which is now a closed establishment holding a solely juvenile population.
There are 9 residential units,
- Blything – a 42-bed unit holding prisoners on their first night and during induction; it also contains medical use rooms for residents who need in-room toilets.
- Hoxon – an 82-bed normal location residential unit
- Stow – a 76-bed unit normal location residential unit
- Bosmere – an 80-bed normal location residential unit
- Wilforde – a 72-bed normal location residential unit; also holds prisoners with disabilities
- Samforde –a 15-bed unit holding prisoners over the age of 50
- Threadling – a self-contained bungalow for long-term prisoners preparing for release
- Cosford – a 72-bed normal location residential unit
- Mutford: a 43-bed unit housing most of the paid workers and other outworkers