Authorities have shut down construction of a waterfront mansion at Seaforth as they investigate whether an illegal third storey has been added under the watch of a private certifier who regulators unsuccessfully attempted to ban last year.

Workers were forced to down their tools last Thursday at the building site, perched on a cliff face at Seaforth and boasting views over Middle Harbour.

The half-constructed home at Seaforth. The owner says the finished product will only have two storeys. Credit:James Brickwood

At the centre of the fracas is landowner Peter Prasad, who told The Sydney Morning Herald he was “very shocked and surprised” at Northern Beaches Council’s stop-work order and that the “third storey” that has been the subject of multiple complaints is actually an “under-house”.

Overseeing the build is private certifier Stanly Spyrou of Dix Gardner, who regulators have reprimanded in connection to 20 developments across Sydney since 2016, including a unit block at Alexandria that was the subject of demolition orders.