
There is a 9.5 foot wide townhouse in Manhattan at 75 1/2 Bedford Street [1], that is supposedly the narrowest.
The townhouse with the smallest width I have inspected was 12 feet wide and that seemed pretty tight. Figuring about 1 foot for wall thickness on either side in the house I inspected, the occupant is left with 10 feet to move around in.
….and a narrow house in Seattle, WA San Francisco, CA [3].