Game of Thrones itself was based in the Titanic or Paint Hall Studios in Belfast, Northern Ireland, although overseas shoots ranged from Iceland to Morocco (and even, briefly, Los Angeles). HBO announced in March that they would not be returning to the Paint Hall, which surprised a lot of commentators. House of the Dragon, set during the Dance of Dragons, a brutal civil war fought between different off-shoots of House Targaryen using dragons, utilises many of the same locations as Game of Thrones, including King's Landing, the Red Keep, Harrenhal and Dragonstone, the sets for which were located at the Paint Hall. However, these sets were mostly destroyed during the filming of Game of Thrones' finale, and those which survived had to be torn down to make room for the sets for the first spinoff pilot, The Longest Night, which never got past a pilot order.
In addition, location filming for those locations ranged widely, with Malta and the city of Dubrovnik in Croatia both standing in for the exteriors of King's Landing, whilst some scenes on Dragonstone were shot in Spain and others in Northern Ireland. With House of the Dragon also scouting overseas locations, it seems possible that those locations will be used again, with just the set location in the UK changing.
Leavesden is a solid choice with economic benefits, since the studio complex is owned by HBO's parent company Time Warner. Films shot at Leavesden include the Harry Potter series, Sleep Hollow, The Phantom Menace, GoldenEye, Kingsman, Inception, Justice League, Warner Brothers, Spider-Man: Far From Home and The Batman. The studio is also notably further south than the Paint Hall in Belfast with a more temperate climate and predictable weather.
Production of House of the Dragon is expected to start in January and run through the summer, to debut in early 2022. Casting is currently underway, with Paddy Considine recently cast as King Viserys I Targaryen.