What is the ruling on praying in a place such as a shop or an isolated room in a company where I do not work, without consulting its owners?

Praying in a place where you know its owners permit you to do so is permissible, whether the permission is explicit or understood from custom and common practice. However, if you know that they do not permit it, then praying there is not allowed. As for praying in a usurped place, scholars have differed between its invalidity and its validity accompanied by sin. It is better to search for a permissible place to pray. If you are compelled, praying in the street is better than praying in an impermissible place.