Block B at Bletchley Park

Block B at Bletchley Park

Block B was built in 1942 by HM Office of Works, adapted in consultation with GCCS (Government Code & Cypher School) from a standard MOW Temporary Office Design. It is a steel-framed building with pre-cast concrete floors and roofs, painted Fletton brick walls, and metal windows.

The building is two storeys with a basement, featuring a higher entrance block at its west end. It has a dog-leg plan with spurs extending south, east of the main entrance, and to the north, and is attached to Block A to the west. An imposing boiler house chimney rises within the east range. The entrance is distinguished by fluted pilasters and an upswept canopy. The exterior displays plain brick surfaces with regular fenestration, predominantly twelve-pane rectangular metal casement windows, though those along the south end of the southern spur were later altered by the insertion of taller openings. Concrete bands mark the first floor and roof levels. A later single-storey timber-clad extension to the north is of no architectural interest.

The interior has been considerably altered. The entrance lobby was enlarged in the 1950s through demolition of wartime rooms and now leads to an open staircase with tubular steel railings. A smaller staircase exists at the north end of the east range. Originally, each floor consisted of a spine corridor with offices flanking it on each side, or WCs along the north side of the main range. The centre and western parts of the ground and first floors have been opened out to create display areas. The original layout can still be sensed through exposed steel and concrete beams, under which the original plasterboard partitions were positioned. Some individual rooms remain at the east end, though many date from the 1950 conversion to study bedroom use, which employed clay brick construction for walls. The arrangements of wartime structures, particularly regarding pantry and store facilities, can still be discerned in areas that have not undergone recent change, evident through variations in fenestration and the survival of materials from different periods.

Block B was conceived in mid-1941 as an extension to the overcrowded huts and completed in late summer 1942, forming the first wave of purpose-built structures on the site. It was designed to respond to the increased volume of decrypts and the desire to create an effective military intelligence centre. The building originally housed the Naval and Air Sections, with the ground floor used by the Registry for the western European Cryptography Section and other parts occupied by the Japanese Cryptography Section. From late 1943, the Naval Section took over the entire block. After the war, it served first as a National Service Hostel, then from mid-1950 underwent conversion into accommodation for the teacher training college established in 1947. Further modifications took place in the late 1970s when the Civil Aviation Authority occupied the block. Empty since 1993, the building was recently adapted for use as a museum in 2004.

Bletchley Park became celebrated for its crucial contribution to Allied victory in the Second World War, particularly in breaking the German Enigma code and contributing to success in the Battle of the Atlantic. Block B played an important role in this achievement. The building's principal importance is historical, though its physical survival reflecting the scale of operations at Bletchley is also significant. It demonstrates the first approach of constructing more permanent buildings on the site using a bespoke design after careful planning, and it maintains a significant relationship to the surrounding lake, landscape, and other surviving wartime buildings. Architecturally, it is an increasingly rare example of a rapidly constructed wartime office building that retains its original crisply functional appearance. However, the interior has been substantially altered, and little of the surviving fabric dates from the crucial period of the block's operational history.