Renovate DDR cottage & tear down fireplace?

Good day,
we own a garden plot (not building land) in Falkensee/Brandenburg, on which stands a GDR prefabricated dacha, probably model B26/E (at least that’s what the previous owner said), and a tool shed.
The dacha has a small bathroom with a shower as an extension as well as a masonry chimney.
The roof is presumably made of asbestos and I am also not sure what materials were originally used in the house.
The house and the shed are protected by preservation rights, but slowly it’s time to thoroughly renovate everything.
However, as far as I know, you are not allowed to simply tear everything down and build a new house of the same size, but you must not alter the load-bearing parts, otherwise the preservation rights lapse.
The neighbors have gradually replaced everything on their houses, so that not much of the original house remains.
Since I don’t know whether I might potentially encounter hazardous materials during the renovation, I wanted to know whom to contact who can tell me what the house is actually made of.
Are there specialists for GDR dachas? What job title should I look for?

Also, I would actually like to tear down the chimney and replace it with a regular stove.
Such a chimney may look nice, but unfortunately it hardly warms the house.
I probably need to ask a regular demolition company for that?
Sorry if these questions are very naive, but so far I have only dealt with rental apartments and my manual skills are okay, but I have never had to deal with houses and roofs before.
I would be very grateful for a hint for the proper procedure or maybe even a recommendation.
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we own a garden plot (not building land) in Falkensee/Brandenburg, on which there is an East German prefabricated dacha, probably model B26/E (at least that’s what the previous owner said) and a tool shed. [...] But since I don’t know whether I might potentially encounter hazardous materials during the renovation, I wanted to find out whom to contact who can tell me what the house is actually made of. Are there specialists for East German dachas? What job title should I search for?

There are, check out expert Wagner (Leipzig). There you will also find a compendium of knowledge about the garden house and bungalow models.

Is there, check out expert Wagner (Leipzig). There you will also find a compendium of knowledge about the arbor and bungalow models.

Thank you very much, that already looks good. I will get in touch on Monday right away.