A call for works to preserve a Pembrokeshire mansion’s Grade-II*-listed ‘shell house’ has been given the go-ahead by county planners.

In an application to Pembrokeshire County Council, Suzannah Fleming of registered charity The Temple Trust sought listed building consent for remedial works to parapet walls, eaves and canopy to reduce water ingress at Cilwendeg Shell House, Cilwendeg Park, Boncath.

The application said: “The purpose of the works is to reduce or prevent moisture ingress which in doing so will maintain the delicate shell interior, prevent the timber roof structure from rotting and maintain the visual appearance of the building.”

An officer recommending approval said: “The Shell House is said to have been built in the 1820s for the Morgan Jones family of Cilwendeg. We do not know for certain who the architect was, but it is worth noting that the 1820s remodelling of Cilwendeg House has been attributed to Edward Haycock of Shrewsbury (1790-1870). Haycock re-modelled nearby Plas Llangoedmor in 1833 for the Vaughan family and later designed nearby Penylan House for the Morgan Jones family in 1852.”

It said the shell house ornamental grotto was ruinous by the late 20th century before being restored in 2004-5.

“In recent years, despite the best efforts of the restoration, the damp Pembrokeshire climate has caused water ingress through the parapets and slate porch canopy,” the report said, adding: “The shell house is constructed from both dressed slabs and rubble blocks of Cilgerran slate and local quartz. The parapets have twelve upright quartz boulders known as the ‘apostles’. The original construction method is indicated from the 2004 plans and photographs.

“There is evidence that the ‘apostles’ are held in place with metal dowels. However, the method of joining the quartz rubble outer face to the slate inner skin is unknown.

“Rather than speculating over the construction method and proposed damp-proofing measures, it is considered appropriate to consent invasive works of investigation which will then inform the most appropriate damp proofing measures. The appropriate damp proofing measures will be approved via discharge of condition.”

It said a damp survey, heritage impact assessment and plans for the exploratory works had been submitted and are considered acceptable in principle.

The application was conditionally approved.