Polite, everyday term for one's landlord or landlady. The さん is part of the word, not an optional honorific.
See also: 大屋 (おおや)
I paid the rent to the landlord.
Our landlady is very kind.
家主 (やぬし) also means landlord but is more formal and often used in legal or contractual contexts.
Standard spelling with honorific さん. The kanji 大家 can also be read たいか (expert, master), but 大家さん is unambiguous.
Alternate spelling using 屋 (shop, house) instead of 家. Less common but still encountered.
From 大 (おお, big/main) + 家 (や, house) + honorific さん. The さん is lexicalized, making it the standard polite form.