Onion Flats began in 1997 with the restoration of a 200-year-old building in Philadelphia’s Old City—marking the start of a practice rooted in craft, innovation, and urban sustainability. By 2005, Tim McDonald and Howard Steinberg opened Onion Flats Architecture to expand its offerings to outside clients, ushering in a new era of integrated design leadership. Over the years in business, the firm broke ground on the region’s first LEED Platinum developments and helped pioneer Passive House and Net-Zero–ready building practices nationwide.

From early design-build experiments to today's client-driven work, Onion Flats Architecture has continuously evolved to deliver thoughtful, high-performance solutions tailored to community needs. The firm remains committed to serving clients through holistic, mission-aligned design—advancing equity, resilience, and replicability in the northeast and beyond.