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.
- Hoxon: Mainly single rooms. Two of the four landings accommodate induction.
- Cosford, Stow and Wilford: Mainly single rooms, with two or three double rooms each.
- Bosmere: Part-enhanced unit. A pre-fabricated ‘quick build’ comprising two linked units accommodating 40 prisoners each.