don’t let chatterboxes spoil things for you…
you need a spatial separation from the granny flat. so quickly brick up the door, that only causes problems, even if lockable. but still have a door lintel installed there. if the desire to rent out ever fades, you can easily install a door there yourself.
the creation costs of the granny flat can be written off, the rental income is probably tax-free for the first few decades. costs that otherwise arise from the apartment can be fully deducted immediately, labor and materials. without limit. important: it must be rented out, or you must demonstrably make efforts to rent it out. purchase incidental costs can also be deducted proportionally, interest can be deducted, insurance proportionally, maintenance contract for building technology proportionally…
if you build a KFW house, you can get double the funding. that goes according to residential units. for a 40+ house that means a solid 30k funding.
important is that you must be able to bill separately. it doesn’t have to be a separate meter in the fuse box for a granny flat. you can also allocate based on square meters. or install an intermediate meter. also think of heat quantity meters for heating! the easiest, however, is to rent furnished and warm. furniture is practically given away on eBay classifieds... consider beforehand if everything has to be double, e.g. own Telekom contract for the tenant. or if you run it through your own (warm incl WLAN) [liability for open Wi-Fi was overturned] saves 1000€ connection costs.
regarding cellar as storage: we have quite a large kitchen, and have stored so many supplies there that we lose track. we barely use storage rooms. is the cellar storage a warm cellar with ventilation and bells and whistles? then you can prospectively annex it to the granny flat.
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ground floor. large passages are great. we have a double-wing door living room/hallway that "disappears" into the wall and a super thin glass sliding door dining room/living room, which is so slim that it isn’t noticeable when open. just an idea.
i would try to fit a shower in the guest bathroom, maybe you can take the cupboard in the kitchen out of the niche and instead run it along the whole wall.
upper floor. children's rooms are smaller than the bedroom. i always find that unfair. you just sleep and have sex there, the children withdraw there to play and learn, they "live" there. can that possibly be reversed? then the bedroom is also above the tendentiously noisy living room, no longer the children's rooms.