The year 1820 in architecture involved some significant events.

• Approximate date – Gatcombe Park, Gloucestershire, England, is remodelled by George Basevi (a relative of the owner at this time, economist David Ricardo).

• Charlotte Square, completing the construction of New Town, Edinburgh, Scotland; laid out to the design of Robert Adam (died 1792).
• Bakers Island Light, Salem, Massachusetts, USA.
• Citadelle Laferrière in Haiti, the largest fortification in the Americas.
• Clock tower of Cathedral of St. Mary the Crowned, Gibraltar.
• Eastnor Castle, Herefordshire, England, by Robert Smirke.
• Inveraray Jail and Courthouse, Scotland, by James Gillespie Graham.
• Óbuda Synagogue, Hungary.
• Pont de la Tosca, Andorra.
• 'Stack A' bonded warehouse, Custom House Docks, Dublin, Ireland, by John Rennie the Elder.

• Grand Prix de Rome, architecture: François Villain

• April 21 – Peter Kerr, architect (died 1912)
• date unknown – George Devey, artist and architect (died 1886)

• March 7 – Thomas Baldwin, English surveyor and architect in Bath (born c.1750)
• May 17 – Vincenzo Brenna, Italian architect and painter (born 1747)
• September 3 – Benjamin Latrobe, Neoclassical architect known for the Capitol building (born 1764)
• October 4 – Thomas Hope, architect and house joiner (born 1757)