County Durham Archives. Durham County Record Office is located on the Lower Ground Floor of County Hall, the administrative headquarters of Durham County Council, in Durham City.
Northumberland and Durham FHS covers both Northumberland and County Durham. The Family History Society have a research centre in Percy House in central Newcastle upon Tyne (full address on website), open Monday to Friday from 10 am to 4 pm. Browse the website to see what else the society can provide.
Unfortunately, the Victoria County History for Durham is limited in its coverage. It only includes the cathedral city of Durham, including an account of the cathedral itself. Also covers Stockton Ward in the south-east of the county (bordering North Yorkshire) including Hartlepool and Stockton on Tees.