Morgen wollen wir die Wohnung besichtigen, die in der Wohnungsanzeige beschrieben wird.

Morgen means tomorrow, and German often puts a time expression first when it wants to set the scene.

So:

• Morgen wollen wir ... = Tomorrow we want to ...

This first position is very common in German. When something other than the subject comes first, the conjugated verb still has to stay in second position:

• Morgen
  • wollen
    • wir ...

That is why it is Morgen wollen wir, not Morgen wir wollen.

wollen is the infinitive to want.

Here, wollen is conjugated for wir:

• ich will
• du willst
• er/sie/es will
• wir wollen
• ihr wollt
• sie/Sie wollen

So wir wollen means we want.

In this sentence:

• Morgen wollen wir die Wohnung besichtigen = Tomorrow we want to view the apartment

German often uses wollen + infinitive just like English uses want to + verb.

Because die Wohnung is the direct object of besichtigen.

The verb besichtigen takes an object: you view something.

So:

• wir besichtigen die Wohnung = we are viewing the apartment

Wohnung is a feminine noun:

• nominative: die Wohnung
• accusative: die Wohnung
• dative: der Wohnung
• genitive: der Wohnung

Here it is accusative, but for feminine nouns, nominative and accusative both look like die Wohnung, so there is no visible change.

besichtigen usually means to inspect, to look around, or to view, especially a place such as:

• an apartment
• a house
• a building
• a museum
• a tourist site

In this sentence, die Wohnung besichtigen is a very natural phrase for viewing an apartment, for example before renting it.

It is more specific than just sehen (to see) and more formal/practical than anschauen (to look at).

Because that part is a relative clause, and German always separates relative clauses with commas.

The main clause is:

• Morgen wollen wir die Wohnung besichtigen

The relative clause is:

• die in der Wohnungsanzeige beschrieben wird

This relative clause gives more information about die Wohnung.

So the comma is required in German.

The first die is the article for Wohnung:

• die Wohnung = the apartment

The second die is a relative pronoun:

• die ... beschrieben wird = which/that is described ...

So they do different jobs:

• die = the
• die = which/that

This is very common in German, even though it can look confusing at first.

Because it refers back to die Wohnung, which is:

• singular
• feminine

Relative pronouns have to match the noun they refer to in gender and number.

So since Wohnung is feminine singular, the relative pronoun is die.

Compare:

• der Mann, der ...
• die Frau, die ...
• das Kind, das ...
• die Leute, die ...

Here:

• die Wohnung, die ...

Because die in der Wohnungsanzeige beschrieben wird is a subordinate clause—specifically, a relative clause.

In German subordinate clauses, the conjugated verb usually goes to the end.

So instead of something like English word order:

• that is described in the ad

German gives:

• die in der Wohnungsanzeige beschrieben wird

The finite verb wird comes last.

This is one of the most important word-order rules in German.

Because in can take either the dative or the accusative, depending on meaning.

• dative = location, position, in
• accusative = movement into, into

Here, the apartment is described in the advertisement. That is a location, not movement.

So German uses the dative:

• in der Wohnungsanzeige

Since Wohnungsanzeige is feminine:

• nominative: die Wohnungsanzeige
• accusative: die Wohnungsanzeige
• dative: der Wohnungsanzeige

That is why it becomes der after in.

This is the present passive.

It is built with:

• werden
  • past participle

Here:

• beschrieben = past participle of beschreiben (to describe)
• wird = is

So:

• beschrieben wird = is described

The active version would be something like:

• Die Wohnungsanzeige beschreibt die Wohnung.
• The ad describes the apartment.

The passive version makes the apartment the focus:

• die Wohnung, die in der Wohnungsanzeige beschrieben wird
• the apartment that is described in the ad

In German, when you have a two-part verb form in a subordinate clause, the non-finite part usually comes before the conjugated verb at the end.

So in the passive:

• beschrieben wird

This is normal German clause structure.

Compare:

• main clause: Die Wohnung wird beschrieben.
• relative clause: ..., die beschrieben wird.

In the relative clause, the conjugated verb goes to the end, so the whole verb phrase ends up there too.

Yes, grammatically you could say:

• Morgen wollen wir die Wohnung besichtigen, welche in der Wohnungsanzeige beschrieben wird.

But in normal modern German, die is much more common and natural than welche in relative clauses.

So die is the best choice here.

Yes, it is grammatical and natural.

It sounds like something someone might say when talking about an apartment listing and a planned visit.

A very close variant that is also natural would be:

• Morgen wollen wir uns die Wohnung besichtigen, which is actually not correct standard German.

With besichtigen, you normally say:

• die Wohnung besichtigen

If you used anschauen, then you would say:

• Morgen wollen wir uns die Wohnung anschauen.

So the original sentence is well formed and idiomatic.