I am confused by the use of 上下 in the sentence:
这套房子是上下两层的，楼下是客厅，楼上有三个房间。
This house has two floors, the living room is downstairs, and there are three rooms upstairs.
I understand 上下 as up and down, which does not add any information to the sentence as 两层 two floors can only be at different vertical positions right?
Sometimes we want to be specific, either to disambiguate or add emphasis on the topic
上下两层 - upper and lower floor (两层 can be two floors in one building or two buildings)
左右兩手 - right and left hand (兩手 means one man's two hands, not the same hand twice or two hands from two people)
前后两轮 - front and rear wheel (like on a bicycle, not side by side like on a wheelchair)
Unlikely but possible:
这套房子是两层的 (一层是东楼，一层是西楼)
兩手持刀 (是一人兩手持刀 or 二人各持一刀?)
两轮弯曲 (是单车 or 轮椅的两轮?)
Note:
层 can mean 'floor' or be a classifier for building
The examples above can be used for disambiguation, but using extra words to describe the object can stress its importance in the sentence. It is similar to using an idiom instead of a simple term to stress a topic
You are right that "上下" is redundant in this case. However, you might come across this expression fairly frequently. Because most apartments or houses in China are not duplex. "上下" in this case is for emphasizing the scarcity of this kind of houses.
A better translation of "这套房子是上下两层的" should be: This is a house that has 2 floors.
Chinese and English just have different ways of emphasizing.
"这套房子是上下两层的". This sentence is abbreviated from the longer sentence below:
"这套房子從上到下共有两层", 上 means the topmost level under the roof (頂層); 下 means the lowest level above the foundation (一樓). 上下 can also mean in between the roof and the foundation to count buildings with more than one story:
"这套房子上下共五层" - This house has 5 stories in between the roof and the foundation.
It's true that 上下 is redundant in this sentence. Sometimes redundancy is used to bring emphasis to certain aspect, as explained in Yi Shen's answer.
There's a term for this linguistic phenomenon: pleonasm. Pleonasm at its face value just means redundancy in linguistic expression. Unnecessary redundancy is of course considered bad style, but when used wisely it certainly can add flavor.
Take for example:
- 他们陆续到来
- 他们一前一后陆续到来
一前一后 and 陆续 both mean "one after another". Adding 一前一后 clearly presents the scene more visually.
Another explanation is related to Chengyu (成语), traditional Chinese idioms that mostly consist of four characters, because four-character group is considered rhythmically beautiful.
There're plenty of Chengyu that are constructed by repetition/redundancy, e.g.:
七嘴八舌
挨家挨户
添油加醋
情投意合
You could say pleonasm is somewhat prevalent in Chinese, that I barely notice such fact until writing this answer. And in all these examples, the main purpose of repetition is just to form a four-character group.
Padding 上下 to 两层 in order to make it a four-character expression can effectively render it stylistically more beautiful, formal, thus more presentable.
I'd imagine the owner of the house when giving his guest a room tour would say:
这套房子有两层，楼下是客厅，楼上有三个房间
But a salesperson when presenting a house to his client would probably say:
这套房子是上下两层的，楼下是客厅，楼上有三个房间
The difference is clear, later one is a more formal expression.
Yet another example:
He was so frightened that he went weak in the knees
- 他吓得腿发软
- 他吓得两腿发软
Both are legit translations, but the later definitely sounds better, while the former feels off. Surely people know man has two legs, so 两 (two) is redundant here. It neither means to disambiguate nor to emphasize, it only serves as a padding word to make it four.
Actually the measure word 层 does not necessarily refer to vertical layers. For example compare the following:
- 五层大楼 (vertical, no doubt)
- 两层玻璃窗 (just layers)
Anyway (my very personal, debatable feeling here), why should it be necessary to "add" information? Redundancy (the part of a message that can be eliminated without loss of essential information) may add emphasis or just beauty to a sentence. Do you feel better with "Minimal Chinese" (as in Minimal English)?
Besides, for example, you said:
does not add any information
Does the word "any" really add (any) information to the sentence?
Would the following be the same?
does not add information
上下两层 means 上面有一层，下面有一层.
With 两层 only, it doesn't specify the position of each floor.
It seems to be a bit verbose. It's more of convention here, IMO.
这房子是上下两层的 is the same as 这房子是两层的
No difference, it just depends on the habits of the speaker
No differences, its probably just the way that the speaker talks.
Its like some of your friends going to the Maccas and asking for 'tomato ketchups'.
