Work on the two-storey unit, which will host two specialist adult wards, began two years ago.
The first floor Chine Ward will relocate female patients from a listed building elsewhere on the site which limits modern development, offering improved facilities and an increased capacity of 14 beds.
On the ground floor, Haven Ward – divided into eight-bed male and five-bed female psychiatric intensive care units – will also add to bed numbers on the site, as well as making space elsewhere at St Ann’s for extensive refurbishment and modernisation works, due to be completed over the course of 2027/28.
Dorset HealthCare chief executive, Matthew Bryant, said Chaddesley House will allow the trust to “provide the highest level of care to more local people”.