بالبناية في شقتين فاضيات، وحدة قريبة من الدرج ووحدة جنب شقة جارتي.

Because the two words are doing different jobs.

• بالبناية means in the building.
• في here is the Levantine existential word meaning there is / there are.

So the structure is basically:

بالبناية في شقتين فاضيات
= In the building, there are two vacant apartments.

In this sentence, في is not just the preposition in. It is functioning more like there is/there are.

In Arabic, especially in the present tense, you usually do not say a separate verb for is/are.

So:

• وحدة قريبة من الدرج literally means one near the stairs
• but in natural English it means one is near the stairs

This is very normal in Arabic. The present-tense copula is usually omitted.

شقتين is the dual form of شقة (apartment).

• شقة = one apartment
• شقتين = two apartments

For a native English speaker, the important idea is that Arabic has a special form for two, not just singular and plural.

Also, the spelling changes because the final ة of شقة becomes ت before endings are added:

• شقة
• شقتين

In Levantine Arabic, the dual is very often heard as -ين in everyday speech.

So:

• شقتين = two apartments

In more formal Arabic, you learn a distinction like -ان / -ين, but spoken Levantine usually simplifies things, and -ين is extremely common.

So this is a normal colloquial Levantine form.

Because spoken Levantine often simplifies agreement, especially compared with formal Arabic.

• فاضي / فاضية means empty, free, vacant
• فاضيات is a feminine plural form

Even though شقتين is dual, colloquial speech often uses a plural adjective instead of a special dual adjective. So شقتين فاضيات sounds natural in Levantine.

A more formal style would use something closer to a true dual adjective, but this sentence is clearly colloquial.

Here فاضيات means vacant, empty, or available.

The adjective فاضي can mean different things depending on context:

• for a place: empty / vacant
• for a person’s schedule: free
• for a container or room: empty

So with apartments, the meaning is vacant / unoccupied.

It literally means one in the feminine form, but here it stands for one apartment.

Because شقة is feminine, Arabic uses the feminine one:

• وحدة قريبة من الدرج
• one is near the stairs

Then again:

• وحدة جنب شقة جارتي
• one is next to my neighbor’s apartment

So وحدة ... ووحدة ... means one ... and one ...

Because it refers to وحدة, which stands for one apartment, and apartment (شقة) is feminine.

So the adjective also appears in the feminine form:

• قريب = near (masculine)
• قريبة = near (feminine)

Since the hidden idea is one apartment is near the stairs, the feminine form is used.

جنب means next to, beside, or by.

So:

• جنب شقة جارتي = next to my neighbor’s apartment

This is a very common everyday Levantine word. It is less formal than something like بجانب, but very natural in speech.

جارتي means my female neighbor.

Breakdown:

• جارة = female neighbor
• -ي = my

So:

• جارتي = my female neighbor

That means:

• شقة جارتي = my neighbor’s apartment

If the neighbor were male, you would normally say جاري instead.

Because they are being treated differently in the sentence.

• البناية = the building
  A specific building, known from context.
• الدرج = the stairs
  Also a specific set of stairs in that building.
• شقتين = two apartments
  They are being introduced as unspecified apartments, so they are indefinite.

So the sentence is basically:

In the building, there are two vacant apartments — one near the stairs and one next to my female neighbor’s apartment.