Der Altbau sieht schön aus, aber im Winter ist ein Neubau oft wärmer.

Both are very common German nouns for types of buildings.

• der Altbau = an older building, especially an older apartment building or house
• der Neubau = a new building, or a more modern one

In everyday German, these words often mean more than just old and new:

• Altbau often suggests charm, high ceilings, old architectural details, wooden floors, etc.
• Neubau often suggests modern construction, better insulation, more efficient heating, and newer materials.

So the sentence contrasts beautiful old-style buildings with warmer modern buildings in winter.

This is about the article choice, not about a grammar rule specific to these words.

• der Altbau = the old building
• ein Neubau = a new building

German uses articles much like English here:

• der Altbau points to the idea of an old building as a type, or a specific old building in context
• ein Neubau means a new building / any new building

So the sentence is saying something like:

• The old building looks nice, but in winter a new building is often warmer.

The speaker is comparing one category with another, and the indefinite article in ein Neubau makes the second part sound more general.

Because the verb is aussehen = to look / to appear.

This is a separable verb:

• infinitive: aussehen
• present tense: sieht ... aus

So in a main clause, the main verb part goes to position 2, and the separable prefix goes to the end:

• Der Altbau sieht schön aus.

Compare:

• Ich sehe müde aus. = I look tired.
• Das Haus sieht modern aus. = The house looks modern.

In the infinitive, it stays together:

• Der Altbau kann schön aussehen.

Because schön here is not directly describing a noun. It describes how something looks.

In Der Altbau sieht schön aus, schön functions like an adjective used with a verb, similar to English:

• The building looks beautiful.

After verbs like these, German usually uses the basic adjective form:

• sein → Das Haus ist schön.
• werden → Das Haus wird schön.
• aussehen → Das Haus sieht schön aus.

But when the adjective comes before a noun, it takes an ending:

• ein schöner Altbau
• der schöne Altbau

So:

• Der Altbau sieht schön aus. ✅
• Der schöne Altbau ✅

German main clauses normally follow the V2 rule: the conjugated verb is in second position.

First clause:

• Der Altbau = position 1
• sieht = position 2
• schön
• aus

Second clause:

• im Winter = position 1
• ist = position 2
• ein Neubau
• oft
• wärmer

So even though the sentence starts the second clause with im Winter, the verb still comes second.

This is a core German word-order rule.

No. Aber is a coordinating conjunction, so it does not force the verb to the end.

That means the clause after aber keeps normal main-clause word order:

• ..., aber im Winter ist ein Neubau oft wärmer.

Compare with subordinating conjunctions like weil or dass, which do send the verb to the end:

• ..., weil ein Neubau im Winter oft wärmer ist.

So:

• aber → normal word order
• weil, dass, obwohl, wenn → verb goes to the end

Im is a contraction of in dem:

• in dem Winter → im Winter

German often uses this contraction with seasons, months, and other time expressions.

So:

• im Winter = in winter / in the winter
• im Sommer = in summer
• im Januar = in January

It is the standard, natural form here.

Wärmer is the comparative form of warm:

• warm = warm
• wärmer = warmer
• am wärmsten = warmest

German often forms the comparative by adding -er, and many one-syllable adjectives with a, o, u often take an umlaut:

• alt → älter
• jung → jünger
• warm → wärmer

So ein Neubau ist oft wärmer means a new building is often warmer.

Because oft is an adverb meaning often, and here it modifies the whole statement ist wärmer.

The sentence structure is very natural:

• ein Neubau ist oft wärmer

This means:

• a new building is often warmer

You will often see adverbs like oft, meistens, manchmal, wahrscheinlich in this middle area of the clause.

For example:

• Ein Neubau ist oft teuer.
• Das Haus ist meistens ruhig.
• Die Wohnung ist manchmal kalt.

There are other possible word orders in German, but oft wärmer is the most neutral and natural here.

Yes. That is also correct.

Both are grammatical:

• Im Winter ist ein Neubau oft wärmer.
• Ein Neubau ist im Winter oft wärmer.

The difference is emphasis:

• Im Winter ... puts the time first, so it highlights the contrast with winter conditions.
• Ein Neubau ... puts the building type first.

German is flexible because the verb still stays in second position.

Yes, there is a small but useful difference.

• schön sein = to be beautiful
• schön aussehen = to look beautiful

sein sounds more direct and absolute. aussehen focuses on appearance.

So:

• Der Altbau ist schön. = The old building is beautiful.
• Der Altbau sieht schön aus. = The old building looks beautiful.

In many contexts, they are very close in meaning, but aussehen is specifically about how something appears to the eye.

They are common everyday words, especially in housing and real-estate contexts, but they are not limited only to advertisements.

Very often:

• Altbau refers to an older building, especially one built in an older architectural style
• Neubau refers to a newer or newly built one

In Germany and Austria, these words are very common in apartment hunting, because they suggest typical features:

Altbau

• character
• older design
• sometimes less efficient heating
• sometimes poorer insulation

Neubau

• modern layout
• better insulation
• often warmer in winter
• usually more energy-efficient

So in this sentence, the contrast is very natural and culturally typical.