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Listed Building
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Ordnance survey map of H BLOCK AT BLETCHLEY PARK
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Statutory Address:

The building or site itself may lie within the boundary of more than one authority.

Milton Keynes (Unitary Authority)
West Bletchley
National Grid Reference:
SP 86335 34057


721/0/10012 H Block at Bletchley Park 17-DEC-04

GV II Block H. 1944. Ministry of Works and Planning, for the Government Code & Cypher school. MATERIALS: reinforced concrete with hollow clay block walls; corrugated asbestos roof. Boiler house built of Fletton brick. Mainly timber framed windows, hung from above: some refenestration has occurred. PLAN: inverted T-shape. Off the narrow northern spur on the west side is a lavatory annexe. EXTERIOR: single storey. South range comprises a taller boiler house with water tower and chimney, flanked by machine rooms to east and west. These comprise pairs of Standard Huts placed alongside eachother, with a valley roof above. Continuous strips of windows at upper ground floor height. Northern range, 26 bays in length. INTERIOR: the machine rooms at the southern end have been sub-divided: they formerly had posts supporting the valley roof along the middle, creating large open spaces housing the computers. The corridor along north range survives, although dividing walls in the rooms along the east side (used in part for the initial analysis of decrypted material) have been repositioned. HISTORY: Block H was the last significant building to be erected at Bletchley Park in WW2. Plans were aproved on 25 May 1944: it was ready for occupation on 17 September 1944. It was built on the western edge of the site as an annexe to the now-demolished Block F (pulled down in 1987), which was located immediately to the east, and its purpose was to work on the decipering of the 'Fish' series of encrypted German teleprinter transmissions. The large machine rooms at the southern end were built to house six 'Colossi' (Nos 6 - 11 at Bletchley Park): this was necessary, given the volume of signals being intercepted, and the pivotal role of Bletchley Park in the overall Allied intelligence-gathering process. The Colossus computer was developed in 1943 by Tommy Flowers, based on designs by Max Newman. First housed in the southern annexe of Block F, Colossus II was accommodated in this purpose-built structure. After 1945 the building was adapted for use as a training centre for the Post Office, and the northern range was extensively altered. The building was opened as a museum in 1994, housing a replica of the Colossus. SUMMARY OF IMPORTANCE: Block H is architecturally undistinguished, being a utility design incorporating a standard Ministry of Works 1942 hut design (see The Builder, 28 August 1942, 176). Nonetheless the building has strong claims to international historical note as being the world's earliest purpose-built building erected specifically for electric computers (Block F, now demolished, included an annexe designed to house computers but Block H was designed from the outset to serve this purpose). The Colossus is regarded as being the world's first programmable electronic digital computer: Bletchley Park witnessed the earliest mass-installation of computers, and is thus a key site in the development of information technology. It also forms part of the ensemble of surviving wartime buildings at Bletchley Park which, through their deciphering of encrypted Axis messages, made a significant contribution to Allied victory in the Second World War. The adjoining brick building to the north-east is excluded from the listing.


Linda Monckton et al, 'Bletchley Park' (English Heritage Historic Buildings Report B/101/2004: 4 vols 2004), esp. II, 540-49; IV, 29 & 75.


The contents of this record have been generated from a legacy data system.

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Books and journals
'The Builder' in 28 August, (1942), 176


This building is listed under the Planning (Listed Buildings and Conservation Areas) Act 1990 as amended for its special architectural or historic interest.

End of official listing

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