New basement under the foundation of an existing house

The idea of building a central underground parking garage for several residential units is understandable. When the metal boxes are underground, there is more space for essential things on the property. The conditions for such a construction are unfavorable, as basements under an existing house are significantly more complex than open underground construction. The approval is questionable. A plausibility check on the costs: Underground parking spaces in larger facilities built with conventional methods currently cost about €30,000 each per a brief research. You are building a smaller facility, say for 20 cars, so traffic areas and access facilities per parking space have a stronger impact. Estimate +60% You are building underneath a building and therefore have more complex technology. Estimate: +80% 30,000 x 1.6 x 1.8 = €86,400 per parking space Times 20 parking spaces = €1,728,000 If you rent the parking space for €120/month, you would have rental income of €24,000 per year without self-use and losses. The ratio does not look very lucrative. All assumptions are without guarantee; it is only a possible approximation.

Both posts belong together. Of course you need a building permit for a basement floor. And of course the basement floor (= underground facilities) is counted towards the floor area ratio. In this respect, the clear answer to your question is: Never will it work.

See Baunutzungsverordnung § 19

The entire property is completely calculated as 4 plots. Therefore, the basement area could be divided among the plots.

Maybe one of the James Bond supervillains can recommend his planners – they manage to convert entire volcanic islands into world-destruction factories without anyone noticing.

That would then be Special Executive for Counterintelligence, Terrorism, Revenge and Extortion (S.P.E.C.T.R.E.) :)

Yes, special is probably the right word. So the house footprint is about 100 sqm and then there are 2000 sqm of garden added. Not everything should be built over (I think then I would also destroy the root system and there are a lot of beautiful fruit trees), but about 40 - 60%

Aha. You want to build a basement without destroying the root system? Sorry, but cutting down the trees is the first act after the building permit. How do you imagine that? Like a kind of tunnel boring machine that puts the basement underneath?

Discussing the pros and cons of a basement? Then it requires a discussion based on the actual planning.
Because it should be known that a basement is usually dark and underground ;-)

No, not necessarily a basement but the possibility of subsequently "sliding" a basement under the house. I find that interesting, and who knows if other builders might also want a basement added later.

(Not me; possibly if a zombie apocalypse is looming on the horizon. ;-) )

No, not necessarily a basement but the possibility of subsequently "inserting" a basement under the house. I find that interesting and who knows, maybe other builders would also like to have a basement added later on.

That’s land. Also not a technical problem. Only a financial one. Digging out completely around the house, cutting all utility lines, putting the house on supports, building a basement underneath, installing new connections, dismantling the supporting elements, and finally receiving a bill that your descendants will have a hard time dealing with. I would estimate that you’ll probably easily end up in the five-digit range per square meter under the house. And you bear the risk that the foundation doesn’t go perfectly, that the house settles again, etc.