Alexander Roos was the son of a German cabinet maker, born and brought up in Rome.
By 1835 he had come to London, where he undertook a number of commissions for houses in Italianate style, including Shrubland in Suffolk.
From 1843 he worked extensively in Scotland, and from 1845 to 1868 in Wales. He laid out much of Cardiff, designing Cathays Park and many houses and villas.