Die Wohnfläche beträgt nur fünfzig Quadratmeter, aber der Balkon ist groß.

Wohnfläche means the usable living area of a home or apartment — basically the indoor space counted as living space.

It is a compound noun:

• wohnen = to live
• Fläche = area / surface

So Wohnfläche is not just any area; it specifically refers to the living space of a property.

Because German nouns have grammatical gender.

• die Wohnfläche is feminine
• der Balkon is masculine

That grammatical gender affects articles and other words connected to the noun.

You usually have to learn the noun together with its article:

• die Wohnfläche
• der Balkon

beträgt is the 3rd person singular form of betragen.

Here, betragen means:

• to amount to
• to come to
• to measure

So:

• Die Wohnfläche beträgt nur fünfzig Quadratmeter
  = The living area amounts to only fifty square meters
  or more naturally in English: The living area is only fifty square meters

German often uses betragen in formal or factual descriptions involving size, amount, cost, distance, and similar measurements.

Examples:

• Die Miete beträgt 900 Euro. = The rent is 900 euros.
• Die Entfernung beträgt 10 Kilometer. = The distance is 10 kilometers.

You often can use ist in everyday German, but beträgt sounds a bit more formal, precise, or factual.

Compare:

• Die Wohnfläche beträgt fünfzig Quadratmeter.
  More formal / typical in property descriptions
• Die Wohnfläche ist fünfzig Quadratmeter groß.
  Also correct, and very common

You would normally not say:

• Die Wohnfläche ist fünfzig Quadratmeter

That sounds incomplete in German. If you use ist, German usually wants groß:

• ist fünfzig Quadratmeter groß

So betragen is a neat way to express measurements directly.

nur means only, and it usually goes near the thing it limits.

Here it limits the size:

• nur fünfzig Quadratmeter = only fifty square meters

So the meaning is that the living area is small in size.

If you move nur, the emphasis can change. In this sentence, the natural place is right before the measurement.

Because Quadratmeter has the same form in both singular and plural.

• ein Quadratmeter
• zwei Quadratmeter
• fünfzig Quadratmeter

This is normal for some measurement words in German.

So after a number, you simply say:

• 10 Quadratmeter
• 50 Quadratmeter
• 100 Quadratmeter

Because groß here is a predicate adjective, not an adjective directly before a noun.

Compare:

• der große Balkon = the big balcony
  Here the adjective comes before the noun, so it takes an ending.
• der Balkon ist groß = the balcony is big
  Here the adjective comes after ist, so it does not take an ending.

This is a very common point in German:

• Das Haus ist klein.
• Die Wohnung ist teuer.
• Der Balkon ist groß.

No adjective ending after sein.

Good question. That depends on the type of connecting word.

aber is a coordinating conjunction, so it does not change normal word order. The second clause keeps standard main-clause order:

• aber der Balkon ist groß

That is:

• aber
  • subject + verb

Compare that with words like deshalb or trotzdem, which are not coordinating conjunctions in the same way and often trigger verb-second inversion:

• Der Balkon ist groß, deshalb ist die Wohnung attraktiv.

So with aber, no special inversion happens.

aber means but and is used for a contrast.

Here the contrast is:

• the living area is small
• the balcony is big

So aber is perfect.

sondern is used after a negation, when the second part corrects or replaces the first idea.

For example:

• Die Wohnfläche beträgt nicht fünfzig, sondern sechzig Quadratmeter.
  = The living area is not fifty but sixty square meters.

In your sentence, there is no direct negation of the first clause, so aber is the correct choice.

Because in German, all nouns are capitalized.

So in this sentence:

• Wohnfläche
• Quadratmeter
• Balkon

are all capitalized because they are nouns.

This is one of the most noticeable spelling differences between English and German.

More or less, but learners often need practice with two parts:

• ü in fünf
• -zig

A rough pronunciation guide:

• fünfzig sounds approximately like FUENF-tsikh in standard German

A few notes:

• ü does not exist in English, so it takes practice.
• In standard German, -zig is usually pronounced with a sound like tsikh, not like English zig.

So fünfzig Quadratmeter is worth practicing as a chunk.

Yes. A very common alternative is:

• Die Wohnfläche ist nur fünfzig Quadratmeter groß, aber der Balkon ist groß.

That version is fully correct.

You could also make the second part a bit more specific stylistically, depending on context, for example:

• Die Wohnfläche beträgt nur fünfzig Quadratmeter, aber dafür ist der Balkon groß.

Adding dafür gives the sense of but on the other hand / but to make up for that.

So the original sentence is natural, but not the only possible phrasing.