Conservatory, Stormont Castle, Stormont Estate, Upper Newtownards Road, Belfast

Attached symmetrical sandstone Gothic Revival glazed conservatory, built circa 1850, forming part of the gardens of Stormont Castle on the Upper Newtownards Road, Belfast. The building is octagonal on plan, faces south onto the castle gardens, and is flanked by linear ranges of lean-to glazed greenhouses. It is attributed to Thomas Turner, son of Richard Turner, who was renowned for his conservatory designs throughout Britain and Ireland.

The structure is built in sandstone ashlar, with each face of the octagon framed by angled buttresses. Octagonal piers rise from the offsets above the parapet wall and are surmounted by tapered and crocketed pinnacles with fleur-de-lys finials. The roofline sits behind a crenellated parapet wall. The glazed roof is constructed in timber, octagonal in plan, and is surmounted by an octagonal glazed lantern topped by a sandstone pineapple finial. Three pointed-arched openings feature crocketed hood mouldings rising to fleur-de-lys finials. Each opening houses bipartite multi-pane timber French doors with a central mullion rising as Y-tracery in the overlight, with interlacing glazing bars. The doors open onto five nosed sandstone steps leading down to a paved front area.

The setting is notable. Immediately in front of the conservatory is a circular pond enclosed by a sandstone wall, with a central decorative stone fountain base supporting a black stone vessel. To the rear, on the other side of the backing wall, are single-storey former estate workers' houses, each with a square-topped doorway and two rectangular sash windows.

Stormont Castle itself was constructed around 1830 for the Reverend John Cleland (1755–1834), rector of Newtownards, who had acquired the Stormont Estate by marrying the daughter of the previous owner. Cleland has been described as a fearsome magistrate and a man of dubious reputation, with much of the wealth that allowed him to develop the estate allegedly ill-gotten. Following his death in 1834, his son Samuel Cleland took over the estate, but was killed in 1842 whilst supervising the demolition of a wall in the grounds. Samuel's widow, Elizabeth, continued to manage the estate until approximately 1860. A major alteration and extension of the mansion was undertaken in 1858, giving the building its current Scots Baronial appearance.

The conservatory does not appear on the first edition Ordnance Survey map of 1834 but is recorded on the second edition of 1860, placing its construction between those two dates. Paul Larmour has suggested a late-1830s construction date, but renovations carried out in 2004 and 2009 found the interior to be distinctly Victorian in character. The Natural Stone Database records that the building was constructed using the same locally quarried Scrabo sandstone used in the 1858 extension to the castle, which would support a construction date of around 1858. If that date is correct, Thomas Turner is the likely architect. According to the Dictionary of Irish Architects, Turner did not design any overtly Gothic buildings but did design a number of gate screens and garden features, and submitted an unsuccessful entry for the design of the Crystal Palace built to house the Great Exhibition of London in 1851. His father, Richard Turner, was also employed as a contractor for Charles Lanyon's glasshouse at the Botanic Gardens.

In Griffith's Valuation of 1860–61 the combined value of Stormont Castle and its outbuildings was set at £350. The conservatory was always valued jointly with the mansion, making it impossible to assess its individual worth. Historically the building was divided into two sections: the southern half served as a greenhouse and outbuilding to the castle, while the northern section was used as workers' cottages.

The First Survey Record of 1985 notes that the glass and timber elements of the conservatory were replaced in the 1930s. The conservatory was listed in 1987. Following the Good Friday Agreement of 1998 and the subsequent St Andrews Agreements of 2006–07, Stormont Castle became the offices of the First Minister and Deputy First Minister, with the conservatory used for state and social functions. An inspection around 2002 found rot within the timbers. In 2004 the single-storey glasshouses were demolished and carefully replaced with the current Victorian-style wings, while the central Gothic sandstone structure was retained. Following this reconstruction, part of the southern section continued to be used as a greenhouse, while the remainder of the building, including the former workers' cottages on the northern side, was converted to incorporate modern office accommodation and a meeting venue. An interior renovation in 2009 installed a new Victorian-style tiled floor in the southern section of the conservatory, carried out by Armatile Architectural.

The conservatory is considered highly significant for its architectural style, proportion, ornamentation, octagonal plan form, quality and survival of its interior, and its setting and group value with Stormont Castle. It is also of historical interest for its authorship, its rarity, and its Northern Ireland and international significance. It shares group value with Stormont Castle and with the former estate workers' houses adjoining its rear wall.