Up to 300 new beds could be added to Calderdale Royal Hospital under an expansion of the site into a centralised A&E centre.
A controversial planned shake-up of NHS services would see the hospital become the main emergency centre for Halifax and Huddersfield.
The hospital in Salterhebble could be expanded from 400 beds to between 615 and 700.
Bed numbers in Huddersfield would be slashed by around 280 as the town’s infirmary is replaced with a smaller 120-bed hospital.
Both towns have “urgent care centres” treating minor ailments under plans by Calderdale and Huddersfield Clinical Commissioning Groups (CCGs).
The plans, designed to tackle a £280m funding gap, have led to safety fears over longer journey times to hospital and a campaign to keep both A&Es open.
But NHS bosses say it is safer to take emergencies to a single A&E with consultants on duty 24 hours a day and quicker access to x-rays and scans.
And they believe it could be the only way to make services affordable.
A draft consultation report said it would cost £470m to make Calderdale Royal Hospital (CRH) the main A&E, compared to £501m if it was Huddersfield.
It said: “After working through these financial considerations we came to the view that the CRH should be proposed as the site for the emergency centre.”
Calderdale Royal would have 615 beds, with the potential to expand that to 700 if needed.
Huddersfield’s new hospital would have 119 beds and ten operating theatres.
The plans are dependent on £291m in funding from the treasury to redevelop Calderdale hospital and build the new site in Huddersfield.
A further £179m is needed to tackle a multi-million pound deficit at the hospitals.
The financial burden includes costly repayments for Calderdale Royal Hospital, built 15 years ago and leased back from the private sector under a Private Finance Initiative (PFI) scheme.
NHS bosses considered building a new hospital and vacating the existing sites in Halifax and Huddersfield, but are unable to do so because of a £200m “break clause” penalty in the PFI contract.
The consultation report said Huddersfield Royal Infirmary was more than 50 years old and needed costly repairs.
It said: “If we were to retain it there would be a need for significant rebuilding as well as £20m a year of backlog maintenance until 2020-21.
“On the other hand we have CRH which is a modern hospital that is only 15 years old and already meets modern day building standards.”
The proposals are about to go out to public consultation and will be discussed on Monday by Calderdale and Kirklees joint health scrutiny committee.
The council watchdog could refer the plans to the health secretary Jeremy Hunt if it is not convinced they are safe.