In 1955, a defence report (
Royal Navy in the years to come, Cmdr. Styles et All, 1955) outlined the need for a class of ship to specifically combat the rise of jet aircraft and the emerging second generation of AShM, whilst the County Class DDG was planned around the Sea Slug SAM, the D.EX (Destroyer-Extra ) Class was planned as a evolving design, having over 1000t extra tons of displacement to fit future updates to systems to ensure a minimal gap in capabilities. More specifically the report outlined emerging service aircraft of the Soviet Union and allies capable of attacking Royal Navy assets from great ranges. Whilst the County Class had the capability to attack these targets, it was recognised early on that such a class would be eventually outclassed. Designing started in earnest by Oct 1955 and by April 1956, the first contracts were awarded to Swan Hunter and Cammel Laird for designations D01 and D05. HMS Lancaster was launched on 7th of Jan 1959, HMS York followed on the 21st of December. The ships cost an average of £16.8M and were deemed less favourable to Admitralty to the Counties who were projected at the time to cost £13M. An interesting feature was the more traditional appearance compared to the Counties, the risk of nuclear fallout was considered less of a priority at the time, however the Operations room was located 6 decks below the bridge. From 1965, further development of the NIGS and Type 985 radar was ordered to outfit the class before 1970.
Designed from the outset as close to the County's operating systems, it shared the exact same conceptual propulsion, armament and rough-layout. The steam and gas system was a satisfactory solution and was continued in the second batch of the Counties. The only difference between the two classes regarding propulsion was that instead of 2 Babcock and Wilcox, there were 4. The ships were good for 33.35kts at standard load but had a cruising speed of only 18kts. The second lot of contracts for a projected D09 and D11 was culled to just D09 in the standard tradition of British defence spending. Launched by Yarrow in 1961, HMS Northumberland rounded out the trio, the ships were immediately used as carrier escorts, fulfilling their envisioned roles as fleet AA defence. Whilst for the most part they gave decent service, their huge operating costs, and issues with the Sea Slug made the ships a political sore point for a while. Eventually the faults with the Sea Slug were ironed out but the ship was only meant to carry the system for the first half of service, the RN was impressed by USN missile systems and had hopes on it's own emerging systems.
As Designed:
Length- 564ft (172.1m)
Beam- 50.5ft (15.39m)
Draught- 6.4m (Hull), 7.9m (Total)
Displacement- 7220t standard
Machinery-
COSAG on 2 shafts;
4× Babcock & Wilcox boilers, geared steam turbines, 60,000 shp (22,000 kW)
4× Metrovick G6 gas turbines, 30,000 shp
Range- 3700nm at 13kts
Electronics- Type 965 Air Search Radar, Type 293Q Target Indication , Type 901 Target Guidance, Type 147 Sonar bottom profiling, Type 174 Sonar search, HACs
Armament- 2x twin 4.5"/45 (11.4 cm) QF Mark V, 4x twin 40mm Bofors, L/60 Mk 5 AA, 2x 20mm Oerlikon MG, Sea-Slug SAM, 2x triple Mk IX 21" Torpedoes
Airwing- 2x Westland WS-51 Dragonfly
Complement- 489 (34 officers and 455 enlisted)
Vosper Thornycroft was given the contract to design the refit of the Lancaster Class which was back-typed to Type 52, lead to Cammel Laird being given the contract for all 3 three ships. HMS York was first however a fire in the drydock not a week into work destroyed 40% of the ship and it was found that she was structurally damaged beyond economical repair and was scrapped in 1970. Lancaster was refitted by 1969 and Northumberland was finished at the end of the same year. It took nearly 18 months of working up to get the ships back into full service, working out various issues with their NIGs and training up the crews in the use of new systems. HMS Northumberland was available to fill the role of Fleet AA Escort, she was tasked with the defence of the carrier force, freeing up other escort units however her and her sister were dreadfully under-armed in surface warfare and a modification to ship Exocet and two goalkeeper CIWS were proposed in 1984. However the two ships with their ageing machinery, large complements, inability to hangar mid/heavy helicopters and unique armament meant that in 1987 both were retired, arguably replaced by Batch III Type 42's
Post refit:
Length- 566.5ft (172.72m)
Beam- 50.5ft (15.39m)
Draught- 6.4m (Hull), 7.9m (Total)
Displacement- 7340t standard
Machinery-
COSAG on 2 shafts;
4× Babcock & Wilcox boilers, geared steam turbines, 60,000 shp (22,000 kW)
4× Metrovick G6 gas turbines, 30,000 shp
Range- 3650nm at 13kts
Electronics- Type 985 Air-search Type 293Q Target Indication , Type 909 Target Guidance, Type 162m Sonar bottom profiling, Type 174 Sonar search, GWS24 Target Guidance
Armament- 1x twin 4.5"/45 (11.4 cm) QF Mark V, 2x Seacat Quad Lancher, NIGS SAM, 2x triple Mk IX 21" Torpedoes
Airwing- 2x Westland Wasp
Complement- 432 (31 officers and 401 enlisted)
). 
