Hulland East Yorkshire Hospitals trust has bought a private Nuffield hospital to avoid contracting out services to the independent sector.
The trust said the new building, which will cost£10m to buy and make immediate improvements to, would save it money.
Chief executive Stephen Greep said: "The major element will be that it enables us not to have to use the private sector."
He said the purchase, an unusual move for an NHS organisation, was an "opportunistic" decision taken after Nuffield opted to sell. It is also taking on the existing staff.
The trust will transfer its ear, nose and throat and audiology services from Hull Royal Infirmiary to the hospital and breast surgery from its Castle Hill hospital, which is on the same site.
This would enable the trust to vacate and demolish older wards at the Royal Infirmiary.
"The advantage to us is not on the Castle Hill site, it is being able to bring forward our estate development plans on our Hull Royal Infirmary site," Mr Greep said. "It is on one of our major hospital sites, so it was an opportunity we could not miss."
The move was not directly target-driven, he added. "It is not specifically related to 18 weeks, though it does mean an increase in capacity."
Carrying out increased private work was not "in the plan" for the trust, said Mr Greep. "We are not expecting to see major revenue gains," he said.
In April East Kent Hospitals trust bought the company that ran a 22-bed wing on the same site as one of its premises. The company remained independent, however, with profits returned to the trust.