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Most detailed definition I could find:

> 床下とは  
>  建物の床の下。えん（縁）のした。

Picture from google:

[![enter image description
here](https://i.stack.imgur.com/Yd3J0.jpg)](https://i.stack.imgur.com/Yd3J0.jpg)

Example:

> その大雨で床下30センチまで水につかった。  
>  The heavy rain flooded my house up to 30cm below the floorboards.

However, what I don't understand from the above is:

Does the water rise within 30cm distance of the wooden floorboards (upper
limit) or 30cm above the cement floor (lower limit, like on the picture)? Does
床下 refer to the whole space? In any case, **what exactly happens in the
example sentence?**
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meaning
What is 床下? (picture attached)
I am pretty sure that 床下30センチ in your sentence means 30 centimeters above the
ground, with additional information that it is below the floor level. It does
not mean 30 centimeters below the floor level (that is, the distance from the
floor level was 30 centimeters), which would be 床の30センチ下.

床下 means the space under the floor, as in 床下に収納スペースがある家 (a house with an
underfloor storage).

* * *

**Note/Warning/Excuse.** I was pretty sure when I posted this answer, but I am
no longer sure. I admit that I could not find evidence that my interpretation
was correct. broccoli forest’s answer proves that at least some people
interpret it differently from what I wrote above. Maybe my interpretation is
simply wrong, or maybe the sentence is ambiguous.
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Upper limit.

Though 床下【ゆかした】 stands as a noun by itself, it literally means "under-floor".
Therefore, 床下30センチ always means "-30cm from the floor level". 床 in this phrase
is understood as the floorboards you live upon. If you want to tell you have
30cm deep water in the space between the base and the floorboards, you have to
say 床下 **に** 30センチ (so that you grammatically treat 床下 as a noun).

Similarly, 床上【ゆかうえ】 is used for "above the floor level" as well.

Or more generally, there are lots of words with ～上/下 that means "(distance)
above/below _X_ ".

  * 桁上【けたうえ】/桁下【けたした】 _above/below the girder_ (see the pic)
  * 股上【またがみ】/股下【またした】 (of bottom wear) _rise / below the crotch_
  * 地上【ちじょう】/地下【ちか】 _above/below ground_
  * 氷点下【ひょうてんか】 _below freezing_ (= minus degree Celsius)

* * *

[![enter image description
here](https://i.stack.imgur.com/TSuhx.jpg)](https://i.stack.imgur.com/TSuhx.jpg)

* * *

**EDIT**

"床下 + number" is possibly open to two ways of interpretation (see the
comments)... But at least I've found [a J-E dictionary
entry](http://dictionary.goo.ne.jp/je/77294/meaning/m0u/) that says:

> 床下10センチまで水につかった  
>  The flood waters rose to within ten centimeters of the floor of his house.
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I understand the sentence, “床下30センチまで水につかった” implies that the water below the
floor came up to 30 centimeters high from the ground. I casted this question
to several members of English speaking circle I'm with to monitor other's
view. The answer was unanimously 'the height from the ground.' Reason? It’s
easier to fathom the depth of water from the ground than measuring the length
of gap between the crest and floor level by creeping into 'below-floor.'
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