Are theravada monks allowed to stay in lay people's house? A theravada monk would like to stay in my house for one night. I live with my parents, but  we don't have special room for guest. Should I suggest him to stay in monastery near my house?
Sankha's answer is technically right from an orthodox point of view, but may not be a correct interpretation of the texts. The word the Buddha used is "sahaseyya", which means "sharing a place to sleep", so the rules quoted about roofs and walls are not literal translations, they are extrapolations based on the commentary.

The origin story is what makes me think this rule should be considered less strict; in it, Anuruddha goes to stay in a rest house (āvasathāgāra) and is invited by the owner to sleep in a "bed that is within" (mañcakaṃ abbhantaraṃ). Now, I understand that the commentary gets into an explanation of how even a seven-story palace is considered "sahaseyya", but following the story, it seems likely that the point is not to stay in the same bedroom as the woman. The room where Anuruddha stayed, by the language used, seems to have been an inner room in the rest house that belonged to the owner herself; when he spent the night with her there (teaching her the dhamma, no less), he caused controversy, and instigated the rule - though it seems clear he never laid down, which would be necessary to break the rule.

If Anuruddha had stayed in the outer room, he ostensibly would not have instigated the rule (provided there were no women staying there).

There are other reasons, however, for not staying in a lay person's house, especially that of a person of the opposite gender, and the strict interpretation of the rule is probably for the best - to keep monks in line and to keep lay people from becoming too intimate with the monks, leading to problems on both sides.

My reason for giving this answer is to reassure you about the monk's intentions - many Theravada monks have no problem with staying in the same building as a lay woman, as long as they are not in the same room. I don't personally think it is a good idea, but I'm not convinced that it is breaking any rule, especially in modern times, where our lives are lived inside single buildings that can even house multiple families.