If one called a technician to perform repairs in his home, and he arrived on Shabbat, is one allowed to let him in to do the work?
Clearly, the question pertains to a case where one did not instruct the technician to come specifically on Shabbat. Therefore, one could argue to be lenient and classify this as "Kablanut," in which a worker is contracted on a per-job basis. In general, such arrangements are permitted, because the non-Jew chose to do the work on Shabbat for his own benefit and convenience.
Nevertheless, the Halacha is stringent in this case. Since the work is being carried out on the premises of the Jew, there is the concern of Marit Ayin. Outside observers are likely to assume that the non-Jew was hired in a non-Halachic arrangement.
The Mishna Berura (252:17) cites the Hayeh Adam (Rav Abraham Danzig of Vilna, 1748-1820) who rules that this presents a problem even though all technicians work on a "Kablanut" basis, and no one will think that he was hired by the hour. Nevertheless, people who see the technician working in the Jew’s house may assume that the Jew ordered his services specifically for Shabbat.
Therefore, when ordering technicians, one should tell them not to come on Shabbat. If they do arrive on Shabbat, one must tell them to leave and return after Shabbat.
SUMMARY: One is not permitted to allow a technician to perform repairs in his home on Shabbat.