WHO uses the spaces? WHAT problem is the housing solving? WHERE has it been tried? WHEN? WHY is this useful for Toronto?

Densifying the yellow belt:

Multifamily houses within existing buildings

350 words
WHO uses the spaces?
WHAT problem is the housing solving?
WHERE has it been tried?
WHEN?
WHY is this useful for Toronto?

Intro (150 words):
Densifying the city with affordable housing for lower middle class family dwellings consider the natural environment as well as urban environment, contributing to them both