Halacha forbids hiring day-laborers who receive a daily wage to build for him on Shabbat. (In the case of a contractor, who is paid for the project, rather than by the day, it is technically permissible to allow the building to continue on Shabbat, but the Halachic authorities nevertheless rule that the construction should not be allowed to take place on Shabbat.) The Shulhan Aruch (Orah Haim 244:3; listen to audio recording for precise citation) writes that if a person violated this Halacha, and hired day-laborers to build on Shabbat, it is proper for him not to live in that home. Since the home was built in clear violation of Halacha, he should not move into and reside in the home.
Rav Shemuel Pinhasi (contemporary scholar in Israel), in his work Ve'daber Davar, rules that in such a case, the owner may sell the house to another Jew, and the buyer would then be allowed to move into the house. He explains that this Halacha was enacted as a penalty against only the individual who allowed the home to be built on Shabbat, and not against other people. As such, he may sell the home and the purchaser may certainly then move into the home without any concern.
Hacham Ovadia Yosef, writing in the journal "Yated Ha'me'ir," ruled that even the original owner may reside in the home if otherwise he would suffer a considerable financial loss. If, for example, he has nowhere else to live and no reasonable possibility of obtaining another residence, it is permissible for him to reside in the home built on Shabbat. The Sages did not extend this provision to situations where the individual would suffer financially, and hence under such circumstances he may live in the home despite the fact that it had been built on Shabbat.
Summary: If a home was built by gentile day-laborers, in violation of Halacha, the person who hired the laborers should not reside in the home, unless he would suffer a considerable financial loss should he have to find a different residence. He may sell the property, and the buyer would then be permitted to reside in the home.
