Emperor Leopold I enlarged the crypt in 1657 in the area under the nave of the church and his son Emperor Joseph I extended it further westward and built another mausoleum chamber and a chapel to the east in 1710, but awkwardly, beginning the vault that his brother Emperor Charles VI continued westward in 1720 that extends under the chancel and the apse choir above. For the first time, a well-known architect (Lukas von Hildebrandt) was involved with an enlargement of the crypt.