If by any chance it rains tomorrow, I'll stay home.
Literal
By-any-chance tomorrow rain [subject-が] fall-if house [at-に] exist.
万一 ('by any chance, in the unlikely event') adds a low-probability hedge to the conditional. It's more emphatic than もし — it implies the speaker considers rain unlikely but is planning just in case. A useful set adverb for hedged conditionals.