The new hospital on Villas Road will be built on land in between Ashworth hospital and HMP Kennnet in Maghull.
It will replace the existing Scott Clinic in St Helens, which is now considered no longer fit for purpose.
The proposed 123-bed unit will treat men and women with mental health problems who have come through the courts. The project will include eight wards around a therapy and well being hub, sport centre, cafe and security area.
If the plans are approved Kier could start construction by April next year.
The Maghull hospital has been designed by architect IBI Group with engineering services provide by WSP/PB.