Lancashire Care NHS Foundation Trust wants to build a 30-bed low secure unit on land adjoining the medium secure unit Guild Lodge.
News of the planned development falls in the same week as another public inquiry regarding the future development of the former mental hospital - which could see up to 650 new homes and 9,000sqm of light industry on the site - gets underway.
The land on which the proposed new unit would be built was originally designated as an area for light industrial development when the first Whittingham redevelopment plans were drawn up, until the NHS decided to retain it for their own use.
The same land also came under the spotlight when leader of Preston Council, Coun Ken Hudson, appealed for one of the trust's four new mental hospitals planned for Lancashire be built there, saying mental health was accepted within the Whittingham community.
Plans for the new state-of-the-art low secure unit - which will include the transfer of an 18 bed unit and services currently provided at Ridge Lea in Lancaster - are to be submitted to Preston Council in mid-August.
Subject to planning permission being granted, the trust hopes to start work in early January 2009 with completion the following year.
The trust say the building would be specifically designed to offer a modern, therapeutic environment using the latest design ideas, and the views of those working in the service and people receiving care will be built into the design.
Chief Executive Finlay Robertson says: "We have promised the people of Lancashire that we will significantly improve the buildings in which mental health care is provided. This state of the art development is a great example of what we intend to do across the county."
The unit is part of the trust's £150 million plans to replace all existing inpatient units across the county over the next decade including the four new mental hospitals, the chosen central Lancashire mental hospital site being Ribbleton Hospital, not Whittingham.
A trust spokesperson confirmed that once the 30 bed low secure unit was built at Guild Lodge there would still be some land left over which could potentially be used for the development of future services.
When questioned as to whether or not the mental health rehabilitation unit The Hermitage - which lies close to sporting, leisure and education facilities planned for the site - would be incorporated within the new low secure unit following appeals from the community, she said: "The development of the low secure unit on this land has no impact on the location of the Hermitage."
She also said the trust was currently in discussions with Lancashire's primary care trusts and local authorities to establish which of the four new mental hospitals should be built first.
Lancashire Care NHS Foundation Trust provides mental health services for adults and older people and substance misuse services for a population of around 1.4 million people, covering the whole of the county and employing around 3,500 staff at more than 100 sites.