I have written, many times over the years, about small-scale commercial uses in residential neighborhoods. Here in Toronto, they are generally not permitted. The small convenience stores and bodegas that remain are often legal non-conforming uses.
Today I came across a great example from Calgary. I don't know if it was done on an as-of-right basis or if variances were needed, but it is an example of small-scale commercial on a site that used to be low-rise residential.
Here is the before (from street view):
