The General Medical Council has taken a further 10,000 sq ft in Allied London's 3 Hardman Street in addition to the original 50,000 sq ft confirmed in January.
Steve Jones, head of facilities at the GMC, said: "The additional space in the new premises will allow us to accommodate additional GMC services in Manchester."
The General Medical Council will move into the Spinningfields building later in the year from Bruntwood's St James's Buildings on Oxford Road.
Mike Ingall, chief executive of Allied London, added: "We have continued to successfully lease buildings throughout the last 12 months due to location, specification and our ability to be flexible. It is great to see existing tenants using Spinningfields, and in particular 3 Hardman Street, for different parts of their organisations."
The GMC registers doctors to practise medicine in the UK and aims to protect, promote and maintain the health and safety of the public by ensuring proper standards in the practice of medicine.