I was mainly an 'academic' Geotechnical Engineer, specializing in Rock Mechanics, but I did have 'a hand in' the normal business. That's mainly foundations, drainage, and retaining walls. The current Toronto housing market is such that there are 10 offers for every house, and nobody can get a professional inspection. One reasonable guy says he's offered 20 times and never got a house. People are crazy in this market and getting into serious trouble. Mold in the basement? No drainage? Furnace about to blow up? It's all sold 'as is'.
Near my place is a place they are just fixing up for sale. Nobody has looked at the rotten retaining wall all around. You don't see it in 10 minutes looking from the lot. There was no consequence of failure, until these new people put on an extension, and the 'deck quality' foundations are in the zone of influence.
The general rule of thumb for bearing influence is 45 degree angle (wall about 4 feet high). This soil is a particularly horrible silt, with considerable frost-heave. All my stone garden retaining walls, about a foot high are tilting after 20 years. I did insist with my interlocking driveway to put in extra gravel and cloth, so it has remained stable, Can't say the same for all the other neighbourhood driveways, with the usual 6 inches of stone. If I was doing another retaining wall, I would do what Quebec is doing now and backfill with structural styrofoam.
Being a safety issue, I had informed the City. I think they are very harried right now, so maybe they'll never get to it before it sells again. Naturally, having no lateral resistance on the foundations is also an earthquake issue, but Toronto likes to have Italian-grade seismic capacity. "It will never happen here." they say.