20 Aldermanbury

20 Aldermanbury is a Grade II listed building, a purpose-built insurance office with a richly decorated interior executed in a bold interwar style.

The main front to Aldermanbury presents an advanced range of three-window bays to the right beneath a shaped gable with finial and central four-light mullioned and transomed window. The third floor has three three-light windows above a wrapping cornice. An oriel occupies the central bay at first and second floors, beneath which are the arms of the Chartered Insurance Institute—a salamander for fire, a wheat sheaf for life, anchors for marine and chains for accident. To the left stands the entrance, featuring a two-bay arcaded porch at ground floor with a Romanesque-style column capital. Within the porch is a timber oriel and timber double doors with overlight. An inscription to the right of the entrance records the opening of the building by King George V and Queen Mary in 1934. At first floor is a six-light window of single lights, and at second floor a four-light window beside a longer two-light window. The third floor displays a three-light window above the cornice, while the fourth floor has a three-bay colonnaded porch with two dormers above. The north return contains a chimney stack and is blank; it adjoins a 1964 extension of no special interest.

The south elevation is defined by mullioned and transomed windows, those serving the Great Hall extending through two storeys and those at the second and third floors nearly square in proportion. Above these the roof steps back, featuring large windows—some replaced—and dormers hidden from Aldermanbury by the large gable. The east elevation has an advanced porch inscribed with the building's name over the door, surmounted by a strapwork balustrade with finials. Above stands the Great Hall window and a small balcony at second floor on corbelled brackets.

The interior is exceptionally fine. The entrance hall features a groin vaulted ceiling and three rounded arch openings, that to the south containing heraldic glass in the doors and relief wood carving above. The ground floor room is the Great Hall, which displays moulded beams on scrolled corbels, half-height pegged plank and muntin panelling with linenfold pilasters, and smooth coursed ashlar walls above. The dais stands at the east end and the gallery at the west end, the latter fitted with metalwork balustrade and a sympathetic circa 2000 screen below. Large lights of three tiers in Deco style fill the room; the windows carry heraldic glass representing the different insurance companies. The main stair hall contains two-thirds height plank and muntin style panelling and similar double doors with semi-circular heads bearing relief wood carving leading into the Great Hall. The stair is an open well in grand 17th-century style with closed string, turned balusters and heavy newels with acorn finials. The stair well is dado-panelled, above which boards carry a large collection of fire plaques from around the world. The upper floors and basement have similar panelling at the landings.

At second floor is the main chamber, which matches the Great Hall in its panelling, heraldic glass, beams and lights, though on a smaller scale. The adjoining anteroom serves as the fire plaque museum, lined with grey and green marble cornice and architraves. Above are murals by C. Walter Hodges representing the four types of insurance: Fire depicted through a dramatic scene of the Great Fire of London; Marine by a New World scene with a shipwreck; Life by a rural collage comprising a funeral procession, the planting of crops and a young family; and Accident by two unusual 1930s scenes of a car and work accident. Trompe l'oeuil brickwork integrates the scenes into the architecture. The main stair has a late 20th-century skylight over a moulded base, and a secondary stair serves the rear. The offices are modernised with no features of special interest.

Metal railings to the front are set between large stone piers with domed heads. Railings to the rear are marked for 'Trades'.