I would like to know where the houses that were used are located, especially the large, white, cubist looking house with the Art Deco interior decorations.

Most of the houses and other locations used are listed historically on (the website I can't remember, sorry), and they travelled to location for Death on the Nile, Murder in Mesopotamia etc., but for Poirot's apartment building, Whitehaven Mansions, they used Florin Court in Charterhouse Square, London EC1. It's still there today too, as you can see in the video. If you want a more detailed idea of the location, check out HerculePoirotCentral for maps.

link

Hope this is what you wanted! If you want more information, just leave another question and I can probably help you. :)