heya.... there is a
thread about brick RMH in a public space in germany (outdoors). there are some being built in germany. i m not sure if any of them is approved.
EDIT: here s the link
https://permies.com/t/59362/Brick-RMH-public-space
thing is: who checks your fireplace? the official chminey-sweeper (bezirks-schornstein-feger). he has to approve of a fireplace. the law ("feuerstättenverordnung") seems not to be very specific on that.
AFAIK schornsteinfegers can approve of self-built stoves if they like and see these ovens as safe and enviromentally friendly. they might require that the
project is done or supervised by a professional
oven builder. there are lots of masonry-heaters in germany. each is somewhat unique. maybe RHMs would fall into that category.
i think, the schornsteinfeger will check for things like the height and type of the chimney, the ventillation, the clearanc to combustable things etc. he might even measure exhaust gasses, but that would not be a problem with RMH.
concerning home-insurance: i think, if the schornsteinfeger says your stove is safe, they can t argue that.
AFAIK woodstoves need some kind of ceramic-filter that filters particles from the exhaust. which would not be a problem with RMHs, anyway.
i m not sure if it would be wise to talk to the schronsteinfeger about your plans. maybe it would be wise to say that you like to build one in a
greenhouse or outdoor-patio. and then ask: "oh, by the way, what about this inside? would these be possible?". a demonstration of a working RMH outside might help. but i don t think that it would guarantee successful approval.