Donal,
We have had limited success installing rocket
heaters with a permit. The two easiest ways are 1) if you need something small (around 30k btu) the Liberator and a small mass bench is a fantastic option fully UL listed and EPA approved. I have installed them in several states. That setup comfortably heats my 1100 square foot manufactured (doublewide) home in Cleveland Ohio. 2) a rocket heater can be designed and built to fit masonry heater code IF your
local inspector is willing to go through the process and look up a part of the code they may never have looked at rather than just demand a UL listed woodstove be installed. I will be up your way teaching Rocket Mass Heaters at the Mother Earth News Fair in September if you want to organize a build.
You can see videos of several rocket heater installs on my youtube page
https://www.youtube.com/c/UncleMud