Newlyn tidal observatory and part of the south pier

The building comprises a tidal observatory constructed for the Ordnance Survey between 1913 and 1915, along with a section of the south pier. The observatory is constructed of concrete blocks with a flat, felt-covered roof and has a rectangular plan. It is a single storey building with a single room and a small lobby on the south side set back from the west elevation. The original design features canted corners and a shallow cornice to the roof. Large, square timber-framed windows with steel double-shutters are on the west and north elevations, with the north window featuring small, top-hung brass-framed casements. Access is provided through sheet-steel doors in the west elevation of the lobby, which contains a plaque commemorating the bicentenary of the Ordnance Survey. The eastern wall of the observatory forms part of the pier parapet.

Internally, the door to the observatory is simply boarded and set within an iron frame. A stilling well trap-door is roughly central, constructed of timber sheets with holes for the float in the stilling well shaft located within the south pier. A Munro tidal gauge is positioned at an angle above the trapdoor on a cast iron base bolted to the floor, and a pulley/hook is attached to the ceiling above, originally used for the steel dipping tape. A square, brass Observatory Bench Mark is set into the floor adjacent to the well shaft.

The pier extension is constructed with concrete faced in granite blocks, tied together with steel bars, and sits on a concrete raft on a rock foundation, two feet below the seabed. The concrete blocks were manufactured by Harvey & Co of Hayle at a yard on the nearby beach, using Portland cement and materials supplied by the St Ives and Penlee Stone Company.

It is noted that the south pier lighthouse is not considered to be of special architectural or historic interest, however proposed works that could affect the listed building's character may still require listed building consent.