Yes they are pretty common. They are stowage compartments on the floor behind the last row of seats in front of a bulkhead. About 2ft high and a foot deep. Used for all sorts of things like pax oxy bottles and fire ext etc.
411A From United States of America, joined Nov 2001, 1826 posts, RR: 9 Reply 6, posted (6 years 6 months 2 weeks 1 day 7 hours ago) and read 1613 times:
There are really all kinds of 'doghouses'.
On the Douglas DC-6B as an example, it is the area beneath the main cabin floor through which pneumatic, hydraulic, electrical lines are routed.
Another doghouse is for the fuel tank interconnecting lines.