In the series, Poirot's office is part of his home, located in the fictional Whitehaven Mansions. The real building is known as Florin Court. It is located on Charterhouse Square in London, on the eastern side of the park. Florin Court is just north of the Museum of London at London Wall. For those not familiar with London, Charterhouse Square is west of Goswell Road/Aldersgate Street & one block north of Beech Street. The address is Florin Court, Charterhouse Square, London EC1.
The filming location for Agatha Christie's Hercule Poirot's office in the David Suchet version is actually a set created for the show and not a real building in London. The exterior shots were done at a studio, and there is no specific address for the physical office as it is part of a production set.