Proposals already exist for a £17m museum at Auckland Castle in Bishop Auckland, where religious paintings by Spanish artist Francisco Zurbaran hang.
Now planning permission for additional schemes costing more than £7m is to be sought before the end of the year.
They include a £3.5m gallery dedicated to Spanish art and a £1.5m study area.
Auckland Castle Trust, which owns the former Bishop's home, said it hoped the redevelopment would attract 130,000 visitors a year.
Building work is due to begin in 2015 and take about three years.
The castle's current biggest attractions are the Zurbaran paintings, which have hung in the castle for 250 years.
Trust chief executive David Ronn said: "Our aim in all this is to be a catalyst for the regeneration of Bishop Auckland and especially the historic and very fine Market Square.
"The work of the Trust over the last two years has created 40 jobs, with 80% of those employed living within 20 miles of the castle and there are many more to come."
Further development plans have been announced for a County Durham castle which was home to the Bishop of Durham for 800 years.