20 Lower Castle Street, Glenarm, Ballymena, Co Antrim, BT44 0AT

No. 20 Lower Castle Street is a plain, two-storey rendered terraced house dating from around the 1840s. It forms part of a short block of three largely identical properties set on the east side of Lower Castle Street, a short lane running southward from Castle Street itself. The front elevation faces roughly west and is asymmetrical. On the ground floor to the left is a timber sheeted door with a five-pane rectangular fanlight. To its right is a sash window with Georgian-pattern glazing (six panes over six), and there are two similar sash windows on the first floor. The front façade is finished in painted roughcast. A relatively recent projecting street lamp of traditional style is fixed to the first floor on the right-hand side.

To the right side of the rear elevation there is a large two-storey gabled return, apparently of relatively recent construction (possibly around the 1980s). To the left of this, and joined to it, is a single-storey lean-to structure that appears to incorporate a boiler house, with a tall water tank on top. On the first floor of the south face of the return there is a window with a modern frame. Only a small section of the first floor of the main rear façade is now exposed, where there is a window matching that of the return. The rear elevation is finished in roughcast, with the exposed east face of the lean-to partly in breeze block. The roof is gabled and slated, with a Velux window to the rear. There are rendered chimneystacks to the north and south. Rainwater goods are a mix of cast iron and PVC.

The building has been reconstructed and altered, though some internal detail survives. Despite these changes, the house retains importance for the contribution it makes to the immediate townscape.

Lower Castle Street sits off Castle Street, the shortest of Glenarm's four original main streets, which runs westward from the junction of Toberwine and Altmore Streets to the bridge over the Glenarm River. Castle Street was historically part of the main road from Larne, a route of possibly medieval origin that wound northwestward through The Vennel and over the bridge before curving north toward what is now the Straidkilly Road. The earliest leases in the Antrim Papers relating to plots along the street date from 1711, though at least one building — the old parish church, situated at the southwest end of the street where the former schoolhouse now stands — was already in existence by 1683. The reputed site of the 13th-century Bisset castle, thought to have been a tower house, lay at the northeast end of the street; the still-surviving former courthouse, believed to have been standing at least since the 1750s, is thought to incorporate part of its ruins. The bridge at the west end was erected in 1682 — prior to which travellers forded the river — but had to be largely rebuilt in 1713 after flood damage. The street was originally known as Bridge Street, a name that appears to have persisted until the mid-19th century.

John O'Hara's map of 1779, the earliest surviving plan of Glenarm, shows the street fully developed on both sides, and the rows of properties visible on the Ordnance Survey map of 1832 correspond to much the same extent as today. Evidence from the 1859 valuation suggests that most of the buildings shown in 1832 are those that survive today, with the short terrace on Lower Castle Street having appeared around 1835 to 1840. O'Hara's map clearly illustrates the importance of Castle Street as part of the main northward route from Larne, with the road skirting the grounds of Glenarm Castle on the west side of the river. In the early 19th century this arrangement was fundamentally altered: the old road was superseded by the new Coast Road and the new Glenarm Bridge, built in 1813 at the north end of the village, and the castle grounds were enclosed. This process, complete by the mid-1820s, led to a gradual decline in the street's status. It is perhaps telling that whereas a notable figure such as Lord Antrim's agent could be found living in Castle Street in the late 18th century, by the mid-1830s the agent had moved to the south end of the newly widened and considerably grander Altmore Street.

The short terrace to which No. 20 belongs is recorded in the 1859 valuation but does not appear on the 1832 Ordnance Survey map, with the valuation evidence pointing to a construction date of around 1835 to 1840. In 1859 the terrace was in the hands of a Mrs Eliza Boyd, with this particular property occupied by a Patrick McCambridge and an Alice McKeon.