This little building was likely purpose-built around 1965 as an apartment/office duplex. At least I've got a newspaper ad for that year describing it as such, further claiming it to be "new"*. The spot was at one time (1912) the backyard stable (green arrow) of a Mr John Burke, horse trainer, who lived at 262 Nepean.
The current footprint is roughly square — about 20' by 20' gives us all of 400'sq per floor. A mailbox count suggest three apartments.
* A building of like configuration is clearly visible on a 1933 aerial photo, but looks to be gone by 1958. That's the sort of thing that gives time-travelers headaches.
