Die Wohnfläche beträgt nur fünfzig Quadratmeter, aber der Balkon ist größer, als ich erwartet habe.

Wohnfläche means living area or floor space actually used for living in a home or apartment.

It is a feminine noun in German, so it takes the article die in the nominative singular:

• die Wohnfläche

This noun is very common in real-estate German. It is a compound noun:

• wohnen = to live
• Fläche = area / surface

So literally it is something like living-area.

betragen is a common verb used for amounting to, coming to, or measuring.

So:

• Die Wohnfläche beträgt fünfzig Quadratmeter. = The living area is fifty square meters.

German often uses betragen when giving measurements, totals, prices, sums, distances, and so on.

Examples:

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

You may also hear ist in simpler speech, but beträgt sounds especially natural and standard in this kind of factual statement.

After numbers, units of measurement usually stay in their basic form in German:

• fünfzig Quadratmeter
• zwei Kilo
• drei Liter

So German does not add an English-style plural ending here.

Also, Quadratmeter is the normal word for square meter(s), and the singular and plural look the same:

• ein Quadratmeter
• zwei Quadratmeter
• fünfzig Quadratmeter

nur means only or just.

In this sentence:

• Die Wohnfläche beträgt nur fünfzig Quadratmeter = The living area is only fifty square meters

Here, nur modifies fünfzig Quadratmeter, meaning the area is small or limited.

Its position is important: it comes right before the part it emphasizes.

Compare:

• nur fünfzig Quadratmeter = only fifty square meters
• Der Balkon ist nur größer ... = the balcony is only bigger ...
  (different meaning)

So in your sentence, nur is placed exactly where a German speaker would normally put it.

Here, größer is being used predicatively, after the verb ist:

• Der Balkon ist größer. = The balcony is bigger.

When German adjectives come after sein, werden, or bleiben, they do not take adjective endings.

Compare:

• der größere Balkon = the bigger balcony
  (attributive adjective, before the noun, so it takes an ending)
• Der Balkon ist größer. = The balcony is bigger
  (predicative adjective, no ending)

So größer is correct here.

größer is the comparative form of groß:

• groß = big / large
• größer = bigger / larger
• am größten = biggest / largest

The vowel changes from o to ö, which is common in some German comparatives.

Other examples:

• jung → jünger
• alt → älter
• lang → länger

So größer is just the normal comparative form of groß.

In standard German, als is used after a comparative:

• größer als = bigger than
• kleiner als = smaller than
• teurer als = more expensive than

So:

• Der Balkon ist größer, als ich erwartet habe. = The balcony is bigger than I expected.

This is one of the most important comparison patterns in German:

• so ... wie = as ... as
• -er ... als = -er ... than

Examples:

• Er ist so groß wie ich.
  = He is as tall as I am.
• Er ist größer als ich.
  = He is taller than I am.

Because als ich erwartet habe is a subordinate clause.

In German, subordinate clauses are generally separated from the main clause by a comma.

So the structure is:

• aber der Balkon ist größer = main clause
• als ich erwartet habe = subordinate comparison clause

That is why the comma is required in standard writing.

This is because als ich erwartet habe is a subordinate clause, and in German subordinate clauses, the conjugated verb goes to the end.

The clause uses the Perfekt tense of erwarten:

• ich habe erwartet = I expected

But in a subordinate clause, it becomes:

• ..., als ich erwartet habe.

So the normal word order changes because of the subordinate clause.

Compare:

• Ich habe das nicht erwartet.
  = I did not expect that.
  (main clause)
• ..., als ich das erwartet habe.
  = ... than I expected.
  (subordinate clause)

It could, but German often leaves out an object when it is clear from context.

Here, the speaker means something like:

• Der Balkon ist größer, als ich (ihn / es) erwartet habe.

But because the meaning is obvious, German naturally omits the pronoun.

This is very common. English does something similar in sentences like:

• It’s bigger than I expected.

You do not have to say expected it in English, and German works similarly here.

Because der Balkon is the subject of the second main clause:

• aber der Balkon ist größer

The subject is in the nominative case, so masculine Balkon takes der.

Compare:

• der Balkon = nominative, subject
• den Balkon = accusative, direct object

Examples:

• Der Balkon ist groß.
  = The balcony is big.
• Ich sehe den Balkon.
  = I see the balcony.

So here der Balkon is correct because the balcony is the thing that is bigger.

aber means but.

It connects the two main ideas:

• Die Wohnfläche beträgt nur fünfzig Quadratmeter
  = The living area is only fifty square meters
• aber der Balkon ist größer, als ich erwartet habe
  = but the balcony is bigger than I expected

So the speaker is contrasting two facts:

1. the apartment itself is rather small
2. the balcony is surprisingly large

That contrast is exactly what aber expresses.

It is mostly in the present tense, but the comparison clause uses the Perfekt form erwartet habe.

Breakdown:

• Die Wohnfläche beträgt ... = present
• der Balkon ist größer ... = present
• als ich erwartet habe = literally than I have expected, but in natural English this is usually translated as than I expected

German often uses Perfekt where English simply uses the past.

So even though habe erwartet looks like have expected, the natural translation here is:

• The balcony is bigger than I expected.

Yes. The sentence has two main clauses joined by aber, and then a subordinate comparison clause.

1. Die Wohnfläche beträgt nur fünfzig Quadratmeter

   • Die Wohnfläche = subject
   • beträgt = verb
   • nur fünfzig Quadratmeter = measurement / complement

2. aber der Balkon ist größer

   • aber = but
   • der Balkon = subject
   • ist = verb
   • größer = comparative adjective

3. als ich erwartet habe

   • als = than
   • ich = subject
   • erwartet habe = verb at the end because it is a subordinate clause

So the full logic is:

• The apartment is small,
• but the balcony is larger
• than I had expected / than I expected.