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.

Arthur Howard, 74, and his 75-year-old wife Emma, from East Kilbride, South Lanarkshire, had the winning ticket for the Mega Friday draw on 29 January.
Mr Howard said he "went into complete shock" after phoning the claims line and hearing the words "Congratulations, you're a millionaire.
He now plans to trade in his taxi for a new Audi car.
Mr Howard said: "It has taken a while to sink in, I even had to phone the claim line twice just to make sure it was the real thing.
"When I first phoned and heard the words 'congratulations you're a millionaire' I went in to complete shock and told the call handler I would have to hang up and give her a call back."
Mr Howard added: "My youngest son came over to me as he was worried something was wrong as all the colour had drained from my face.
"I told him and he wouldn't believe me so we called the claim line back, putting it on loud speaker this time, and she said it again 'you are a millionaire'. It really was quite something."
A £3m dedicated unit to help new mothers with serious mental health problems is to be set up in Norwich.
[NEXT_CONCEPT]
A Scottish couple are celebrating after winning £1m and all-expenses paid break to Dubai in a EuroMillions draw.