When a church in Toronto's west end was converted into affordable housing nearly 15 years ago, the group behind the project was already thinking ahead.

Andrea Adams, the executive director of the non-profit developer St. Clare's, said she was "daydreaming" about what could be built on the yard next to the 20-unit building on Ossington Avenue.

She was eventually introduced to Assembly Corp., a company that builds mass timber modular housing, around the same time that the city was looking for propo…