Modular housing development has long been a fringe part of the U.S. market, primarily limited to lower-budget or emergency housing. Now, the country’s largest apartment operator is trying to change that… Proponents say this type of building can be completed faster using fewer workers and with materials that can be purchased at a bulk discount, which can reduce overall costs. Even so, modular remains only a small portion of the overall construction market, reflecting a number of challenges from the cost of transporting pieces to difficulties with financing and regulatory approval. But its use is steadily growing. With the construction workforce shrinking and costs rising, the efficiency gains of modular are gaining traction with mainstream developers. [A subscription to the Wall Street Journal is required to read this article.]