What to do (renovation with new extension or demolition and new construction)?

A part of an inheritance community wants to keep (at least) part of the parents’/grandparents’ house and live in it themselves and, firstly, renovate and modernize this old house and, secondly, add an independent (new) extension, a kind of semi-detached house, which can be rented out (with one to three apartments, not yet clear). That is the very rough outline. The house was built in 1925/26, extended upward in 1936, expanded by an extension in 1964/65. Hipped roof (create an additional half floor by enlargement? According to the building authority, probably possible.), old section has a basement, slightly damp at one corner, extension has only a crawl space without foundation, no connection to the old basement. Demolition of the extension and the mentioned addition of a new one. Is there a recommendation in the Berlin area for a reputable company (no DIY or shoddy companies) that can and wants to handle this professionally and solidly from A to Z? "Solid" or made of wood, we are flexible. And at least the new part should meet KFW 55 standards, that’s what we thought. Unfortunately, the companies want to demolish everything immediately in order to place their finished houses on it. Does anyone have advice?

So image- and plan-less, your story is of little use. You will find plenty of threads here about [Erbengemeinschaften], [Doppelhaushälften], and the like using the search function. Additionally, there is an entire moderated section solely about experiences with construction companies, where you will surely find something with "Berlin" or "Brandenburg".