Conversation so far:
What was Basel in relation to Hans Holbein the Younger?
Basel was where Hans Holbein the Younger worked as a young artist and where he bought a house in St Johanns-Vorstadt.
What did Hans Holbein the Younger use the Basel house for?
Hans Holbein presumably returned home to preserve his citizenship, since he had been granted only a two-year leave of absence to England.

Question: What was significant about the house?
Self-contained rewrite: What was significant about Hans Holbein the Younger's house in Basel?