The Norfolk and Suffolk NHS Trust eight-bed unit at Hellesdon Hospital is due to open to patients next year.
Mothers and their newborn babies will be able to stay together during psychiatric care, the trust said.
Referrals will come from Norfolk, Suffolk , Cambridgeshire and other parts of the country and the unit will work closely with local medical groups.
Mental health illnesses such as postnatal depression and severe anxiety will be treated as well as postpartum psychosis - a mental health condition which can occur in women who have just given birth and can cause hallucinations and delusions and can severely disrupt perception, thinking, emotions and behaviour.
Jessica Bannister, from Norwich, received mental health crisis treatment with Norfolk and Suffolk NHS Foundation Trust (NSFT) when she suffered this condition following the birth of her son, Albert, three years ago.
She then had to travel to Hackney, in London, for specialist inpatient treatment.
"The immediate care that I received from NSFT when I went into crisis was literally life-saving, offering an urgent short-term safe haven, and for that I will always be grateful.
"But there were no local facilities to treat my condition in a specialised environment," said the author and playwright.
"These units are incredible places - the humanity, love and support you receive is phenomenal. The dedication of the staff to your recovery and wellness, along with the love they give to your child, is fantastic."
She has written a drama documentary for BBC Radio 4 called Mama Courage, due to be broadcast on 12 May at 14:15 BST.
A £3m dedicated unit to help new mothers with serious mental health problems is to be set up in Norwich.