18 Clarendon Street, Londonderry, County Londonderry, BT48 7ET

18 Clarendon Street is an early Victorian mid-terrace townhouse of two bays and three storeys with an attic level, built in red brick in the Georgian style around 1856. It sits on the north side of Clarendon Street within the Clarendon Street Conservation Area, approximately 0.1 miles west of the River Foyle, in the townland of Edenballymore. The building is rectangular on plan with a projecting return to the rear, and its principal elevation faces south.

The roof is pitched and clad in natural slate with two rooflights to the front, black clay ridge tiles to both the main roof and the return, and a slender red brick chimney stack rising from the east side, centred on the ridge and topped with six clay pots. Cast iron half-round guttering on rise and fall brackets runs along the front elevation, with cast iron rainwater goods to the rear.

The south-facing front elevation is laid in Flemish bond brickwork. Window openings have cut brick flat arches with six-over-six timber sliding sash windows set within painted cement rendered reveals and painted sills. The entrance doorway is to the left and takes the form of a three-centred arched opening with a moulded surround, a moulded entablature supported by a pair of Doric columns, a simple radial fanlight, and a four-panelled timber door. To the right of the door are two diminished windows at ground floor level, with two windows on each of the first and second floors — notably, the upper floor windows are not aligned with those below. The west side elevation is abutted by the adjoining No. 20 Clarendon Street, and the east side elevation is abutted by No. 16 Clarendon Street.

The north rear elevation is rendered, three storeys in height, with an attic dormer to the left and a four-storey return built at half-landing height to the right. The exposed section to the left has a single six-over-six timber sliding sash window at ground, first, and second floor levels, and a six-over-three timber sliding sash to the dormer. The north face of the rear return has a single six-over-six timber sliding sash window right of centre on the first and second floors, a six-over-three sliding sash on the third floor, and an obscured ground floor. The west face of the return is blank. The east face has a square-headed door opening at ground floor with a timber sheeted door and a plain glazed overlight, and a single diminutive two-over-two timber sliding sash window at first and second floor levels, aligned over the left jamb of the door below.

The property is bounded to the front by Clarendon Street, behind a low rendered plinth wall with replacement cast iron railings enclosing a small hard-surfaced area, which also contains a bootscraper. To the rear is an enclosed yard. At the north end of the site stands a two-storey stone-built outbuilding with a pitched natural slate roof, also accessible from a laneway to the north.

No. 18 was built together with the adjoining Nos. 12–16 and 20 Clarendon Street, forming the earliest group of townhouses constructed along the street. It shares group value with Nos. 6–16 and 20–48 Clarendon Street, a terrace of eleven similar Georgian-style early to mid-Victorian townhouses built over an eight-year period lining the north side of the street. Much of the building's original character and detailing survives.

Clarendon Street, Londonderry, was laid out in the early Victorian period as part of the most ambitious town planning project undertaken in the city since the construction of the walled city in 1613–19. The geometric street pattern followed the model of Georgian street development also seen on Great James Street and Queen Street, and was driven by significant growth in the economy and population of the city during the mid-19th century. The historian John Hume notes that between 1825 and 1850 reconstruction within the city walls took place alongside the first development of housing outside the walls at Bogside and Edenballymore. The first edition Ordnance Survey map of 1830 records the Clarendon Street area as rural hinterland with few structures; by that date the city's streets extended no further than Waterloo Place, Abbey Street, and William Street. The only major buildings north of the walls in the early 19th century were isolated institutions such as the Londonderry Infirmary, the Lunatic Asylum, and Foyle College. The only building in the area predating the early Victorian development is Foyle Cottage, a Regency house of around 1815. In his Annals of Derry, published in 1847, Robert Simpson recorded that the district later covered by Great James Street, William Street, Little James Street, and surrounding lanes had originally been meadow ground without a house. Development of housing began in the late Georgian period and continued into the Victorian era, with uniform rows of three-storey townhouses establishing a new affluent quarter that became the residence of the city's merchant and professional classes.

The 1847 plan of Londonderry depicted the final layout of the street at least a decade before it was completed, and recorded that it was originally known as Ponsonby Street, named after the Rt. Rev. Richard Ponsonby (1772–1853), Bishop of Derry and Raphoe. By the 1850s the street had been renamed Clarendon Street in honour of George Villiers (1800–1870), the Fourth Earl of Clarendon and Lord Lieutenant of Ireland between 1847 and 1852. The second edition Ordnance Survey map confirms the renaming had occurred by at least 1853. Although the 1847 plan showed Clarendon Street extending from the quay to Francis Street, only the lower section between the Strand Road and Queen Street had been laid out by 1853. Progress was slow: by 1856, Griffith's Valuation recorded only nine dwellings along the entire length of the street. In 1851, Skipton and Miller had advertised building ground on Clarendon Street, Queen Street, and Patrick Street to be let in perpetuity, followed in 1856 by a second drive when additional leases for building ground on the north side of Clarendon Street were advertised.

The original terrace of five three-storey buildings comprising Nos. 12–20 Clarendon Street was leased by the Rev. Henry Wallace, Moderator of the Presbyterian Synod of Ulster in 1839. No. 18 was first recorded in Griffith's Valuation in 1856, when it was individually valued at £30. By 1862 this had fallen to £28, and by 1888 it had been further reduced to £24, at which level it remained until the 1930s. The 1901 census building return describes No. 18 as a first-class building comprising ten inhabited rooms. Between 1931 and 1935, the entire row of Nos. 6–26 Clarendon Street was purchased outright by a Ms. Jennie Steen. The First General Revaluation of 1935 increased the value of No. 18 to £38. Steen continued to own the house through the Second General Revaluation period of 1956–72, by which time the building remained in use as a private dwelling and had been valued at £42 by 1972. In 1978 the Department of the Environment designated Clarendon Street and the surrounding streets a Conservation Area, described as an area of special architectural or historic interest, the character of which it is desirable to preserve or enhance. No. 18 was listed in 1979. Northern Ireland Environment Agency records note that by the 1980s the house had been converted into offices. In 1992 the exterior brickwork was repointed, the roof reslated, and the windows and entrance doors conserved. The building is currently used as office space by the architect's firm Consarc Design Group. Writing in 2013, the architectural historian Calley described Nos. 6–48 Clarendon Street as a delightfully long red brick terrace of the mid-19th century, noting that the buildings are nearly all the same, being three-and-a-half-storey and two-bay, with most ground floors rather inelegantly squeezing in a doorway with two reduced scale window bays, and that the depressed arched recessed timber-framed doorways have simple segmental fanlights and thin Doric columns supporting entablatures. Few of the mid-Victorian townhouses along Clarendon Street remain in residential use; the majority were converted into offices for solicitors, dentists, and accountancy firms during the late 20th century.