noun
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)
大家さんに家賃を払った。
I paid the rent to the landlord.
うちの大家さんはとても親切だ。
Our landlady is very kind.
From 大 (おお, big/main) + 家 (や, house) + honorific さん. The さん is lexicalized, making it the standard polite form.