Smallest possible "mini-house" for residence

Hello everyone,
I have a rather curious question:
How big does a house have to be in order to be approved and to register a residence there? And especially how expensive? I suspect the usual ~3000€ per m² are not realistic for such a tiny house.

Let’s assume the following conditions: Rhineland-Palatinate, flat plot, inner area without a development plan (like neighboring buildings).

Background of the question:
I travel a lot for business & spend most of my vacations with the camper. The "house" would therefore only be used to wash laundry, take a shower, charge the camper’s batteries, and to have an official residence. 10m² would probably be enough. But will that be approved, or does building law know a "too small"?

I travel a lot for business & am usually on the road with the camper during my vacation

It's a pity about the property, others would be happy to be able to build on it. Is there no camper mailbox registration address with parking space, washing machine, and mail forwarding service? A real market gap

There is no such thing as too small. You just have to meet all the building regulations - development and Energy Saving Ordinance/Building Energy Act.

So you need a tiny house. People say from T€ 50/60 for 15 sqm. So your idea is completely overpriced. I would actually just use a mailbox address. Register somewhere with family. Or with friends. Or look for mailbox addresses in forums/marketplaces.

10m² would therefore probably be sufficient

the people with the tiny houses are already very limited in terms of the locations where they can place them

There are certainly campsites that offer permanent pitches for mobile homes or tiny houses.

BUT: Why not just rent a small granny flat with a parking space? Why tie yourself to the burden of owning a home? There is always a lot to do, and it is not cheaper either.

What a shame about the plot, others would be happy to be able to build on it.
Is there no camper mailbox registration address with parking space, washing machine, and mail forwarding service?
A real market gap

I wouldn’t want to take any plot in a new development area or in a bigger town but rather some in the middle of nowhere. There are still areas with < €50/m². Personally, I’m not very worried about taking away a highly sought-after building site from a young family.

There are indeed campsites that offer permanent pitches for mobile homes or tiny houses.

BUT: Why not just rent a small granny flat with a parking space? Why tie yourself down with the burden of home ownership? There is always a lot to do, and it’s not cheaper either.

Permanent pitch rentals are currently in high demand and price-wise also above €2k/year base price + per person fees, etc., and prices are currently rising sharply.

Regarding the granny flat with parking space: that's how I’m doing it currently.

The main reason for the consideration is not that I find a tiny house much cooler than a granny flat. It’s more about the plot/garden. I’m a big fan of ponds. I know they also need maintenance. However, I’m rather of the type that likes "natural" ponds, and I find it great when I can watch over the years how they develop (I used to have a pond).