There are three stages of realizing Dhamma (Magaphala) before stepping to the fourth stage or the final stage, which is the attainment of Nibbana. All those three stages can be attained within being a layperson. If someone fully realized Dhamma and became an Arahant, he will not live the lay life anymore spending time on accumulating worldly pleasures and items. Thus, an Arahant cannot live as a layperson anymore. A person who became an Arahant in his lay life often joins the monkhood quickly. It is clear from this explanation that even a layperson can become an Arahant.