ᐅ New Basement Beneath the Foundation of an Existing House

The idea of building a central underground garage for multiple residential units is understandable. When the parking boxes are located underground, there is more space available on the property for essential uses.
The conditions for such a construction are unfavorable because basements under an existing house are significantly more complex than conventional open excavation.
The feasibility of obtaining building permits / planning permission is questionable.
A rough cost estimate:
Parking spaces in larger facilities built using conventional methods currently cost around €30,000 (about $32,000) per space after a brief search.
You are planning a smaller facility, say for 20 cars, so circulation areas and access facilities will weigh more heavily per parking space. Rough estimate: +60%
You are building under a building and therefore require more complex technology. Rough estimate: +80%
30,000 x 1.6 x 1.8 = €86,400 ($93,000) per parking space
Times 20 parking spaces = €1,728,000 ($1,860,000)
If you can rent the parking space for €120 (about $130) per month, without personal use and vacancies, you would have rental income of €24,000 ($26,000) per year.
This ratio does not look very profitable.
All assumptions are without guarantee; this is only a possible approximation.

Benutzer200 schrieb:

Both posts are related. Of course, you need a building permit / planning permission for a basement level. And of course, the basement (= underground structures) counts towards the floor area ratio. Therefore, the clear answer to your question is: this will never be allowed.

See Building Use Ordinance § 19

The entire site is calculated as consisting of 4 separate plots. Therefore, it would be possible to divide the basement area among the plots.

Myrna_Loy schrieb:

Maybe one of the James Bond supervillains could recommend their planners – they somehow manage to transform entire volcanic islands into world-destroying factories without anyone noticing.

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

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https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/

Chris-Cross schrieb:

Yes, special is probably the right word. So, the house footprint is about 100 square meters (1,076 square feet), and then there are 2,000 square meters (21,528 square feet) of garden. Not everything should be built over (because I think that would also destroy the root systems, and there are a lot of beautiful fruit trees there), but roughly 40 to 60%.

I see. You want to build a basement without damaging the root systems? Sorry, but cutting down the trees is the first step after getting the building permit/planning permission. How do you imagine doing that? Like some kind of tunnel boring machine that installs the basement underneath?

Benutzer200 schrieb:

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

No, not necessarily about a basement itself, but the possibility of adding a basement underneath the house afterwards. I find that interesting, and who knows if other homeowners might also want to add a basement after construction.

(Not me; maybe only if a zombie apocalypse is on the horizon. ;-) )

Pinkiponk schrieb:

No, not necessarily a basement but the possibility to add a basement underneath the house afterwards. I find that interesting, and who knows if other homeowners might also want to add a basement later on.

That depends on the plot. It’s not really a technical issue, just a financial one. You would need to excavate completely around the house, disconnect all utility lines, support the house on jacks, build the basement underneath, reconnect the utilities, remove the temporary supports, and finally receive a bill that your descendants will have a hard time dealing with.

I would estimate that you are easily looking at five-figure costs per square meter (square yard) under the house. Plus, you bear the risk that the foundation might not be perfect, the house could settle again, and so on.